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Bacillus thuringiensis(Bt) transgenic cotton has been planted widely since it was commercialized at 1997 in China.It was observed that resistance to bollworm was reduced with the application of small boll cultivars in China.The objective of the study was to investigate the effects of square size on the contents of Cry1 A insecticidal protein in square during the course of square development.Three experiments were conducted at the Yangzhou University Farm,Yangzhou,China.In 2013 and 2015,two Bt transgenic cotton cultivars(medium in maturity,Gossypium hirsutum L.),Sikang 3(hybrid Bt cultivar) and Sikang 1(conventional Bt cultivar) were compared for square volume,square dry weight and Bt toxin content from15 to 25 days after square appearance(DASP) and from 15-29 DASP at 2013 and 2015 cotton growing seasons,respectively.As expected,the differences between cultivars were significant for all the characteristics.Cultivar with bigger square volume had higher insecticidal protein contents in square.In 2014,leaf cut(LC) and square removal(SR) treatments were applied to Sikang 1 and Sikang 3.The square volume and square dry weight were decreased by LR,but increased by SR.LC significantly reduced and SR significantly enhanced the insecticidal protein contents square.The correlation between square size and square insecticidal protein at 15 and 30 DASP was highly significant and positive(0.88 and 0.751),respectively.Therefore,the results of the study indicated that the square insecticidal protein content was significantly affected by square size.The implications of the observed results in breeding,cultural practices and pest management were discussed.