Objective: ESGO/ESTRO/ESP guidelines recommend that DNA mismatch repair (MMR) proteins or microsatellite instability tests should be performed in all cases of endometrial cancer. This study aims to clarify the relationship of MMR protein deficiency (dMMR) between early and advanced stages of endometrial cancer. Secondary objective is to identify dMMR affecting factors in endometrial cancer. Methods: This cross-sectional study was conducted on endometrial cancer patients who underwent surgery at HRH Princess Maha Chakri Sirindhorn Medical Center, Srinakharinwirot University, between May 2013 and April 2021. Patients with endometrial cancer whose tumor tissue was available for analysis were identified. The expression of MMR proteins was assessed by immunohistochemistry, including MLH1, MSH2, MSH6, and PMS2. Then, the pathological specimens were reviewed. Results: A total of 207 patients with endometrial cancer were assessed for data analysis. MMR deficiency was observed in 92 cases (44.4%). We found patients with dMMR in both the early and advanced stages of endometrial cancer-68/155 cases (43.9%) and 24/52 cases (46.2%), respectively (P = 0.774). Statistically significant differences were found only in myometrial invasion (adjusted prevalence odds ratio 2.35, 95% CI 1.21 to 4.57, P = 0.012). Conclusion: Our study showed no difference in tissue dMMR between early-and advanced-stage endometrial cancer. The dMMR was not associated with improved outcomes in patients with endometrial cancer. Even though ESGO/ESTRO/ESP guidelines recommend the performance of MMR IHC or MSI tests in all endometrial cancer cases, we can select the appropriate patients those categorized as "advanced stage" or "recurrent"-who may gain the most benefits from the immunotherapy modality of treatment.