Mucocele of the appendix is a rare entity, characterized by distention of the lumen due to collection of mucoid material. In this case, our aim is to report a mucocele of appendix with computed tomography (CT) findings. A 46-year old male presented to our department with abdominal pain and decreased appetite. Ultrasonography (US) and CT were performed. A tubular cystic mass was detected with US. On contrast enhanced abdomen CT, a nonenhanced cystic mass, measured 4x9cm, was detected at the right lower quadrant near cecum. Radiologic diagnosis was appendix mucocele. By this diagnosis laparoscopy was performed. A mass located at the right low quadrant near cecum was seen. Because of the big size of mass and possible perforation risk laparoscopy was converted to open laparotomy. Mass was excised without perforation. The pathologic diagnosis was mucinous cystadenoma, and there was no malignity evidence. The postoperative course was unremarkable, so he was discharged at the 3rd day postoperatively. Mucocele should be kept in mind as a differential diagnosis of appendix mass. During the operation, surgeon should be aware of possible risk of perforation and pseudomixoma peritonei.