Background: To explore the efficiency of transvaginal ultrasonography in determining muscular infiltration of phase I endometrial cancer prognosis for radiotherapy. Materials and Methods: A total of 96 phase I endometrial cancer patients who were admitted between March 2018 and March 2020 were enrolled and transvaginal ultrasonography was applied for the diagnosis of muscular infiltration. Demographic variables and radiotherapy outcomes were recorded. We hypnotized that only patients with unfavorable profile of sonography receive pelvic radiotherapy. Results: Patients with muscular infiltration > 1/2 had much thicker endometrium than those with muscular infiltration ≤ 1/2. the sensitivity, specificity, accuracy, positive predictive value and negative predictive value of transvaginal ultrasonography in diagnosis of muscular infiltration of phase I endometrial cancer versus surgery results were 84.9%, 88.9%, 87.5%, 80.0% and 91.8%, while the consistency and Kappa value between the transvaginal ultrasonography and pathological test was 72.2% and 0.580. Positive Likelihood ratio of muscular infiltration more than 1/2 was 2.10 (CI95%: 1.22 to 3.61), meaning that approximately 10 patients of 17 patients with muscular infiltration more than ½ would experience radiotherapy later. Negative Likelihood ratio was 0.63 (CI95%: 0.44 to 0.91), meaning that approximately 10 patients of 15 patients with muscular infiltration less or equal than ½ would not experience radiotherapy later. Conclusion: Muscular infiltration of endometrial cancer in transvaginal ultrasonography could be a reliable, simple, convenient and low-cost method predicting the need for the external beam pelvic radiotherapy as an unfavorable progression in endometrial cancer.