The degree of myometrial invasion (MI) is crucial in the preoperative diagnosis of endometrial cancer (EC) using MRI in terms of therapeutic and prognostic implications. However, several pitfalls should be kept in mind when using this modality. We report a case of EC on a 64-year-old woman, identified preoperatively without MI based on ultrasonography and MRI, implying a low risk of lymph node metastasis; surprisingly, the uterine incision showed the lesion had invaded <50% of the myometrium. Thus, a total laparoscopic hysterectomy and bilateral salpingo-oophorectomy were performed, and histopathologic analysis confirmed that the EC was on stage IA (cancer is in the endometrium only or less than halfway through the myometrium). In our case, thinning myometrium and uterine atrophy due to aging, multiple leiomyomas, previous curettage, and blood clots were all pitfalls for MRI in detecting MI. By detecting tiny or isointense tumors and depicting distinct vascularity of the malignancy in postmenopausal women, functional MRI techniques such as diffusion-weighted imaging (DWI) and dynamic contrast-enhanced MRI (DCE-MRI) can help reduce pitfalls when assessing MI. Clinicians can employ DWI preoperatively, which is more reliable and superior to DCE-MRI in determining tumor areas without contrast injection and perform a postoperative histopathological examination to confirm MI in EC.