A 3-month-old American Cocker spaniel was presented at the Veterinary Teaching Hospital, Chungbuk National University, for examination of urinary tract after dissection of vaginal mass at local clinic before 10 days. Clinical examination of the affected bitch revealed a normal sized vulva in a normal anatomical position with a grossly enlarged clitoris, which contained an os clitoris. On examinations of the genital gland, there were testis, epididymis, ductus deferens and uterus. The histology of both gonads was primarily testis. Seminiferous tubules were divided into many parts by fibrous connective tissue. A small number of spermatogonia was present, but large numbers of Leydig's cells were existed. A normal female karyotype (78, XX) was detected in metaphase spreads obtained from cultured peripheral lymphocytes. Y chromosome specific sequences were not detected in genomic DNA by PCR. After 27 months, the os clitoris was larger than 3-month-old dog and os bone was more calcified than young age. Combining the results of cytogenetic, molecular genetic and histological examinations, the dog was diagnosed as a female hermaphrodite with Sry-negative XX sex reversal.