Background and Objectives: Tumor size is one of the independent factors affecting prognosis of patients with soft tissue sarcoma (STS). We evaluated the significance of tumor size in combination with tumor depth in each histologic grade. Methods: A total of 162 adult patients with localized STS in the extremities and trunk were selected. Patient ages ranged from 15 to 84 (median 46.5) years with a male-to-female ratio of 1.19. Histologic grade of tumors was low in 53 cases, intermediate in 51, and high in 58. Two types of categorization were set, and their significance in predicting the prognosis of patients in each grade was evaluated. In the first category (intermediate grade), tumors were dichotomized at 10 cm: Group A comprised patients with deeply seated tumors measuring >10 cm; Group B comprised patients other than those in Group A. In the second category (high grade), tumors were dichotomized at 5 cm: Group C comprised patients with deeply seated tumors measuring >5 cm; Group D comprised patients other than those in Group C. Results: Categorization was not useful in the prognosis of low grade tumors. In the intermediate grade group, the 5-year survival rate of Group B patients (78%) was higher than in Group A patients (59%) (P < 0.05), showing that dichotomization at 10 cm was useful. In the high grade group, the 5-year survival rate in Group C patients (32%) was lower than in Group D patients (56%), showing that dichotomization at 5 cm was useful. Conclusions: These findings show that tumor size for the prognosis of patients with STS differs according to each histologic grade.