Abstract Objective This study aimed to clarify the clinical significance of tumor volume in endometrial cancer. Methods A total of 667 patients with endometrial cancer who underwent preoperative MRI and surgical treatment including lymphadenectomy were enrolled. As the surrogate marker of actual tumor volume, the volume index was defined as the product of the maximum longitudinal diameter along the uterine axis, the maximum intersecting anteroposterior diameter of the sagittal section image, and the maximum horizontal diameter of the horizontal section image from the MRI data. The volume index was divided into five categories: Group 1 (< 8), Group 2 (8 to < 27), Group 3 (27 to < 64), Group 4 (64 to < 125), and Group 5 (125 or more). The relationships between various clinicopathologic factors and volume index were investigated, and Cox regression analysis was conducted to assess the significance of volume index with respect to prognosis. Results High-risk clinicopathologic findings increased with tumor volume. The lymph node metastasis rate was 3% in Group 1, 9% in Group 2, 17% in Group 3, 25% in Group 4, and 53% in Group 5. Cox regression analysis showed that the volume index (≥ 36) was a prognostic factor (hazard ratio: 2.0, 95% confidence interval: 1.3–3.1) independent of older age (≥ 58 years), high-risk histological grade/subtype, deep myoinvasion, lymph node metastasis, and type of surgery. Conclusion Tumor volume successively reflects the state of disease progression in endometrial cancer. The volume index can give information on both the staged prognosis and surgical management.