Intraoperative frozen section (IFS) on endometrial cancer is an invaluable skill for pathologists-in-training to master. Within limited time constraints, pathologists are expected to determine tumor type, grade, and depth of myometrial invasion. During their training, pathology residents gradually gain experience in handling the majority of cases. However, significant errors can still be seen among senior level trainees. We aimed to improve training effectiveness by evaluating our trainees' performance, identifying common errors, and recommending focused curriculum. Twenty-two residents [postgraduate year (PGY)-1-PGY-4] performed 260 IFS during a 4-yr period. We compared their independent IFS diagnoses with final diagnoses. Overall resident IFS accuracy was 73%. Accuracy for tumor type and depth of myometrial invasion was 80% and 93%, respectively. Two thirds of errors were due to sampling with the rest because of interpretation. Major deficiencies lay in recognizing high-risk histologic types (serous, clear cell, sarcoma) and unconventional myometrial invasion patterns (MELF, adenoma malignum, and adenomyosis-like). Resident IFS errors would theoretically result in suboptimal staging for 32 (12%) patients and unnecessary staging for 1 (0.4%). Overall IFS performance improved as training level increased (76% accuracy for PGY-1 accompanied by PGY-5; 59% for PGY-2; 74% for PGY-3; and 86% for PGY-4). We recommend a dedicated curriculum targeting these difficult yet clinically important entities through review literature and a collection of classic cases demonstrating the diverse morphology variations. Implementing such focused training would greatly improve our trainees' competence on IFS, preparing them to handle a wide variety of cases and situations in future practice.