To describe the morphological and histological changes during the annual reproductive cycle of the gonads of Cyprinion tenuiradius, a total of 341 specimens (102 female and 235 male specimens) were collected monthly from the Rudbal River (in the Qarah Aghaj sub-basin, the Persian Gulf basin), Firuzabad, Fars Province, southern Iran. The randomly sampling was adequate to collect males and females at the immature, mature, spawning-active, and non-active phases of reproduction. Five ovarian and five testicular maturation stages (I–V) were described using macroscopic and light and electron microscopic criteria. Also based on the histological examination, six types of ovarian follicles (A-F) and five types of testicular cells were designated. The largest ovarian follicles were found in the stage V of ovarian developmental stages in females with mean body weight and standard length of 22.27 g and 95.13 mm, respectively. The highest number of sperms was observed in the stage V of testicular developmental stages in males with mean body weight of 25.67 g and mean standard length of 97.83 mm. Based on the percentage of late ovary and testis maturation stages (IV, V) and high frequency of large oocytes and sperms it was concluded that the Qarah Aghaj botak spawns once a year during spring and the beginning of summer from April to July.