Aims Although the relationship between size and density is central to plant ecology, research has focused on the above-ground parts of plants and few studies have examined size-density relationships of below-ground modules. Density-dependent regulation in plants can be viewed not only at population and individual levels, but also at plant part level, since growth in plants is a modular process. Our aims were to (1) investigate size-density relationships at plant part level and at individual plant level for both below- and above-ground parts of plants and (2) compare size-density relationships at the levels of plant parts and individuals. Methods Allium cepa var. proliferum was planted in a completely random design at five plant densities (36, 49, 64, 121 and 225 plants·m–2) and three replicates. Plot size was 5 m × 5 m. At harvest time (July 11), ten plants from each plot were destructively sampled to measure plant height, number of leaves, length of leaf blade, bulb diameter and mean weight per tiller. Each plant was separated into root, bulb, sheath and leaf blade, oven-dried for 120 h at 80 ℃, and then weighed. Regression analysis was conducted to explore the relationship between the characters measured and plant density and the allometric relationships of mean masses of below- and above-ground modules with density at plant part level and individual level. Important findings Relationships between plant density and dry weight of root, bulb, leaf blade and sheath showed negative power functions with different allometric exponents for different plant modules: bulb (–1.14) < leaf blade (–1.03) < root (–0.78) < sheath (–0.49). Masses of below- and above-ground dry matter, as well as the whole plant, were decreased significantly with increased plant density. The allometric exponents were –1.13, –0.95 and –0.98 for below-ground, above-ground and individual plant dry matter, respectively. We concluded that the effect of density-dependent regulation on below-ground modules may be greater than that on above-ground modules. Blow-ground parts may be more closely constrained by plant density in contrast to above-ground parts and the whole plant. Consequently, competition for below-ground resources is predominant in the A. cepa var. proliferum populations.