A 11-year, 9-month-old female Shih-tzu was referred for evaluation of the body conditions. According to thephysical examination and ultrasonography, the left ovarian mass was detected. For the removal of ovarian mass, anovariohysterectomy was performed. Grossly, the ovarian mass was full of hairmatrix on the cut surface of the mass. Histopathological findings revealed a characteristic of teratoma, such as laminated keratines, hair, sebaceous glands,neuron tissue, differentiated bone and cartilage. The ovarian mass was well-differentiated components of 2 germ layers,and diagnosed as a mature teratoma