Background: Malignant transformation arising in mature cystic teratoma (MCT) is one of the most serious complications of MCT. Squamous cell carcinoma is the most common malignant change. Some clinical findings such as advanced age group and large tumor size are significant risk factors of malignant transformation. This study was conducted in order to evaluate characteristics, cell types, treatment and outcome of malignant transformation arising from dermoid cysts in our institution. Materials and Methods: A retrospective chart review was performed. General characteristics, operative data, procedure, operative finding and operative outcome were analyzed. Statistical assessment was performed with SPSS version 17.0, using mean, mode, median and percentage to describe those data. Results: During the 10 years period, 11 cases of malignant transformation from a total of 753 cases (1.46% incidence) of MCT were reviewed. Mean age of the patients was 41.2 years (SD 4.34, range 24-70). The most common presenting symptom was a palpable mass (8 cases; 72.7%). Primary surgical staging was performed in 4 patients (36.4%). Re-staging was conducted in the other 4. Complete cytoreduction was obtained in 45.5% (5 cases) and optimal surgical resection was obtained in 36.4% (4 cases). Mean tumor size was 14.1 cm. (SD 1.55, range 6-20). Squamous cell carcinoma was found in 36.4% (4 cases) and mucinous cancer in the other 4. More than half of them were stage Ia (54.5%, 6 cases). All patients whose stage more than Ia received chemotherapy (45.5%). Mean disease free survival was 5.53 years (1.32, 0.3-10). Conclusion: According to our study, the incidence of malignant transformation was consistent with previous studies. The common malignant transformation histologic types are both squamous and mucinous carcinoma which differed from previous reports. Early detection for early stage disease and optimal surgery are important for long term survival.