Objective: To study the differentiation between leiomyoma (LA) and leiomyosarcoma (LS). Methods: One hundred and ninety cases of LA and LS in gastrointestinal tract were investigated by gross, HE staining, special staining, immunohistochemical staining and electric microscopy. Results: The average value of the diameter of LA was 3.3 cm. Tumor surfaces were smooth. Cut section showed hard texture. Interwall and intracavity type were most commonly found.Density of tumor cells reached 1-2 grade. Tumor cells displayed well-differentiation.Nuclei of tumor cells appeared usually to be thin-stick shape or plump-shape. Mean count of mitotic figure was 0.47/25 HPF. Mitotic activity was not observed in 75%of cases. The average value of the diameter of LS was 8.6 cm. The texture was flesh. The dumbbell-shape was commonly seen. Density of tumor cells reached 2-3 grade. Tumor cells were various in shape, and they displayed less-differentiation. Nuclei of tumor cells were irregular in shape or present with giant nuclei or multinuclei. Mean counting of mitotic figure was 20/25 HPF. Specific staining, immunohistochemical staining and electric microscopy were mainly used for determining the origin of tumor cells.Conclusion: Nine items of diagnostic criteria of LA and LS were proposed, in which mitotic figure counting was the basis of diagnosis of LA and the basis of differentiation of low-degree, moderate-degree and high-degree malignant LS. Immunohistochemical staining and electric microscopy are helpful for determining the origin of smooth muscle tumor.