Abstract: The design of an instrument for measurement of the photosynthesis, respiration and transpiration of plant groups under field conditions is shown. It is made of five components: 1. plant chamber, 2. environment control system, 3. gas passage system, 4. measuring system, and 5. power supply. It also has the necessary instruments for measuring environmental factors. Its practical operation on Northeast China’s Yangcao (Leymus chinensis) grassland (1983—1985) proved that all the design specifications had been met. A preliminary approach to the principle of design, measuring procedure and calculating method is also shown in this paper.