In this study,we used island cotton (Gossypium barbadense L.) variety as experimental materials,which are ‘C6015’ and ‘Xinhai 29’,(high fiber strength experimental group);‘Ba 1248’ and ‘Bima 1’,(low fiber strength experimental group).Activity changes and expression profiles of sucrose synthase (EC 2.4.1.13,Sus) during different development periods were measured by fructose and UDPG colorimetric with qRT-PCR methods in two experimental groups,which differences of fiber strength are obvious.Analysis the relationship between activity changes,expression profiles of Sus and fiber strength.In addition,we discussed the main physiological and molecular mechanisms,which lead to form differences of fiber strength in island cotton.The results showed that:(1)the average strength of ‘C6015’ and ‘Xinhai 29’:47.5 cN·tex^-1 and 44.7 cN·tex^-1,respectively;the average strength of ‘Ba 1248’ and ‘Bima 1’:31.2 cN·tex^-1 and 32.6 cN·tex^-1,respectively.The difference was extremely significant about the average strength in two experimental groups.(2)Activity change features of Sus were all single peak curve appear in 4 island cotton varieties,as well as peak appear more early in low fiber strength experimental group.However,peak and later activity of Sus in high fiber strength experimental group were higher and reached to very significant as compared with the low fiber strength experimental group.(3)High expression of Sus genes were Sus1A,Sus1D,Sus3A,Sus3D,Sus6A,Sus6D and Sus8D,but each member of Sus genes had specific expression during fiber development in island cotton.Expression of Sus3A gene start increased to maximum then droped at initial stage of secondary wall thickening (20 days post anthesis) in two experimental groups.Showed that it emphasized on the role of this period;expression of Sus1A and Sus1D genes happened increased phenomenon at late and final period of secondary wall thickening (30-35 days post anthesis) in high fiber strength experimental group.However,expression of Sus1A and Sus1D genes were very low and did not existence this phenomenon at those periods in low fiber strength experimental group.The results showed that both of them work in these periods.(4)The obvious differences about Sus activity level,relative expression of Sus gene each member,as well as Sus activity maintain time during late stage of secondary wall thickening in two experimental groups showed as high fiber strength experimental group＞low fiber strength experimental group.Furthermore,the differences of Sus activity about high or low was synchronize with the differences of expression in Sus3A,Sus1A and Sus1D genes.Research showed that:Sus3A,Sus1A,Sus1D genes associated with formation and differences of fiber strength.They may key genes,which affect the fiber strength.