Abstract: By measuring axial and sympodial leaf areas in the middle position and leaf area index(LAI),the distribution of 14CO2assimilates on different boll-leaf system levels in three boll-weight cotton(Gossypium hirsutum L.) genotypes were studied by feeding 14CO2 on middle axial leaf at full-blooming stage in 2005.These three different boll-weight cotton genotypes were big-boll-weight genotype(Sumian9108,7.49 g/boll),middle-boll-weight genotype(Xinmian33B,4.62 g/boll) and small-boll-weight genotype(Ceheng,3.98 g/boll).The results indicated that areas of axial and sympodial leaves in the middle position were positively correlated with boll weight.The LAI of small-boll-weight genotype was the biggest among the three cotton genotypes at full-blooming stage,followed by big-boll-weight genotype and middle-boll-weight genotype.The 14C-assimilates were mostly distributed in buds and bolls at boll-leaf system,only a little was distributed in other leaves.The proportion of 14C-assimilates in buds and bolls of big-boll-weight cotton genotype were the biggest among these three cotton genotypes,so big-boll-weight cotton genotype had stronger sink activity than other two cotton genotypes.The 14C-assimilates produced in axial leaf were mostly distributed to corresponding fruit branch.In middle-boll-weight genotype,the 14C-assimilates were mostly distributed in upper fruit branch.Only a little was distributed to the lower fruit branches and main stem point.The three bollweight cotton genotypes differed in distribution tendency of 14C-assimilates in middle axial leaf to corresponding fruit.The 14C-assimilates made by middle axial leaf at full-blooming stage were mostly distributed to bolls in the first position,and big-boll-weight cotton genotype had the biggest distribution proportion,followed by middle-boll-weight genotype and small-boll-weight genotype.As to 14C-assimilates distributed to bolls in second position,the distribution proportion in small-boll-weight genotype cotton was the biggest,followed by middle-boll-weight genotype.