Objective In general, surface ulceration in gastric gastrointestinal stromal tumor (GIST) is considered a malignant feature; however, the mechanism underlying its formation has not been evaluated in detail. In this study, we analyzed the factors involved in ulceration using resected specimens of gastric GIST. Methods A total of 48 samples were retrospectively analyzed. We examined the association of surface ulceration of gastric GIST with the MIB-1 labeling index, mitotic number, tumor size, endoscopic ultrasound (EUS) findings and growth pattern on computed tomography (CT). Results The proportion of men was significantly higher in the ulceration group than in the non-ulceration group (p=0.04146), whereas age was not significantly different between the groups. Tumor was significantly larger in the ulceration group than in the non-ulceration group (p=0.0048). There was no correlation between tumor size and ulcer number. The MIB-1 index was not related to ulceration, nor were EUS findings. The number of mitotic cells tended to be higher in the ulceration group than in the non-ulceration group (p=0.05988). Intraluminal growth pattern was strongly associated with ulceration (p=0.00019). After a multivariate analysis, the growth pattern was the only factor associated with ulceration of gastric GIST. Conclusion Although formation of surface ulceration in gastric GIST was partially associated with the degree of malignancy, the growth pattern was the most important factor associated with ulceration in gastric GIST.