A 20-year-old female underwent a health checkup that revealed an abnormal shadow in the left upper lung field on a chest radiograph. A further examination with chest computed tomography( CT) showed a 25 mm solid tumor in the center of B1+2 in the left upper lobe. A biopsy of the tumor obstructing the left B1+2 was performed during bronchoscopy, and a pathological study diagnosed it as mucoepidermoid carcinoma of the lung. The patient underwent left upper lobectomy and lymph node dissection( ND2a-1). The pathological diagnosis was pT1cN0M0, indicating a predominance of high-grade lesions due to the presence of more solid components, including squamous and intermediate cells, than the mucous components. The nucleoli of these cells were distinct and were accompanied by cellular atypia. While most mucoepidermoid carcinomas of the lung are low-grade lesions, this case contained high-grade lesions, requiring careful postoperative follow-up.