We report the diagnostic utility of diffusion-weighted MR images in primary soft tissue tumors. Diffusion-weighted echo-planar imaging was performed in seventy-seven patients with primary soft tissue tumors (benign: 37 cases, malignant: 40 cases). The apparent diffusion coefficient (ADC) was determined from each image. The average ADC of benign cases was 1.53±0.26, and that of malignant soft tissue tumors was 1.31±0.63. In addition, except for three giant cell tumors of soft parts and 11 myxoid type sarcoma cases, ADC of benign soft tissue tumors was 1.58±0.19, and that of malignant soft tissue tumors was 0.94±0.19. This malignant soft tissue tumor ADC valve was significantly lower than that of the benign tumor group (p<0.0001). This indicates that diffusion-weighted imaging is useful in the differential diagnosis of primary soft tissue tumors.