We report a case of giant Mullerian duct cyst in a 6 month-old-boy with urinary tract infection. A massdisplacing the bladder and prostatic urethra anteriorly was found on the voiding cystourethrogram, and it was aoval shaped retrovesical anechoic cyst on the abdominal ultrasonogram. On MRI, it was a tear-drop shaped cyst ofisosignal intensity with a projection toward the prostatic urethra and located in the midline of vesicorectalspace. Grossly, the cyst has communication with prostatic urethra and both vas deferenses were drained to thecyst. Pathologically it was confirmed as a Mullerrian duct cyst lined with squamous epithelium.