The morphological plasticity of plants occurs due to the changes of modular quantity dynamics, modular patterns and modular morphologies. Climbing plants differ from non-climbing and self-supporting plants in many growth characteristics, which mainly result from biomechanical constraints. The morphology and growth of climbing plants may be affected by external supports to a large extent. External supports are important resources and determine the strategies which climbing plants use to obtain maximum sunlight. The object of this study is to describe the effects of external support diameter on growth, biomass allocation and morphological plasticity of herbaceous climbing plants at different modular levels.Three questions are addressed in this study: 1) Do plants have growth, biomass allocation and morphological plasticity responses at different modular levels on different external support diameters? 2) If so, what is the difference between the various modular levels? 3) What is the meaning of those responses on climbing plants to search and locate the external support? The garden experiment was carried out in the Ecological Garden of Southwest China Normal University with a perennial herbaceous climbing species－Momordica charantia. On 6 May 2000, the seedlings of M． charantia with the same size were planted in 40 figuline containers filled with cultivated soil, one seedling per pot. External supports were made of iron line covered with PVC at the beginning of the experiment. All plants were divided into four groups, and each group had supports with 2 mm, 4 mm, 8 mm, 12 mm and 16 mm in diameter, respectively. Each plant was supplied with the same quantity of water and nutrient during the experiment. From 7 June to 6 July, the individual’s growth conditions and branch numbers were recorded at 10 day intervals. All individuals were harvested at 25 August and divided into main stem and shoot, the following parameters were measured separately: a) internode length, b) internode diameter, c) petiole length, d) single leaf area, e) the number of primary branch and f) second branch and the distance from primary branch to base of main stem. The individuals were separated into root, main stem, shoot, lamina, petiole, flower and fruit, and cleaned by tap water and oven-dried for 72 h at 84 ℃, then weighed separately. The main results of this study were as follows: 1) The climbing efficiency of plants was significantly affected by support diameters with climbing efficiency reducing with increase in support diameter. The climbing growth rate of plants on the 2 mm, 4 mm, 8 mm, 12 mm and 16 mm supports were 3.00, 3.00, 2.13, 0.88 and 0.75, respectively; 2) The changes of support diameters did not affect the timing of shoot occurring but significantly affected the shoot numbers and shoot sites along stem. The first primary shoot of all individuals sprouted between 13 June and 17 June, the means of primary shoot numbers on the 2 mm, 4 mm, 8 mm, 12 mm and 16 mm supports were 12.3,10.0,11.3,14.3 and 14.6, respectively; 3) The individuals had higher sensitivity to the changes of support diameter than those of shoots. Plants showed powerful capacity of searching external supports when they encountered the large diameter supports. In this case, the stem extension slowed, specific stem length enlarged, shoot number and branching ratio increased, shoot length and petiole length prolonged, while R/S ratio decreased. The biomass accumulation of individuals was affected slightly by different support diameters, the biomass of individuals grown on the 2 mm, 4 mm, 8 mm, 12 mm and 16 mm supports were 43.56 g, 47.29 g, 34.32 g, 50.46 g and 31.44 g, respectively; 4) The changes of external support diameters had no significant effect on morphological characteristics and biomass characteristics of shoots; 5) The changes of morphology, biomass allocation and shoot behavior of individuals were the responses of the climbing plants to the availability of external supports, which would help the individuals to enhance the capacity of foraging supports.