Objective To investigate the relationship between Pin1 and CyclinD1 expression and the development of gastrointestinal stromal tu?mor(GIST). Methods The protein and mRNA expression of Pin1 and CyclinD1 in 85 samples of GIST and adjacent non?cancerous tissues were detected by immunohistochemistry and real?time quantitative polymerase chain reaction. Results The expression rate of Pin1 protein in GIST tis?sues(64.7%;55/85)was higher than that in adjacent non?cancerous tissues(26.7%;4/15). Similarly,the expression rate of CyclinD1 protein in GIST tissues(42.3%;36/85)was higher than that in adjacent non?cancerous tissues(6.7%;1/15). The expression of Pin1 and CyclinD1 mRNA in GIST tissues was 7.03 and 5.53 times that in adjacent non?cancerous tissues ,respectively. There was no obvious correlation between the expres?sion of Pin1 and clinicopathological parameters. The expression of CyclinD1 was positively correlated with the grade of NIH and tumor diameter (P<0.05). There was a significant correlation between the expression of Pin1 and CyclinD1 in GIST tissues. Conclusion The expression of both Pin1 and CyclinD1 was up?regulated in GIST tissues. The significant correlation between the expression of Pin1 and CyclinD1 in GIST tissues sug?gests that their synergistic effect promotes carcinogenesis and the development of GIST.