In this study, the reproductive characteristics of the King nase ) Chondrostoma regium ) in Bibi-Sayyedan River of Semirom, Iran, were investigated by a monthly collection from March 2010 to March 2011. For this purpose, 471 specimens (320 females, 138 males and 13 immature) were caught by cast net, sein net and electroshocker. The collected specimens were transferred to the laboratory on ice and biometric measurements were carried out. For the histological studies of the gonads, they were fixed in 10% formalin using histology techniques, hydrated and casted in paraffin, then sectioned transversely at 5-7 µm thickness. The tissue were prepared and stained with Hematoxylin–Eosin. The sex ratio was 1M:2.31F. Age groups of the males and females were 1 to 4 and 1 to 5, respectively. The third year age group was the dominant group in both sexes. The maximum length (TL) and weight of males were 18.1 cm and 64.93 g, those of females were 21.5 cm and 90.53 g, respectively. The length-weight relationship for males was as W= 0.007 L 3.088 (r 2 = 0.981), for females as W= 0.007 L 3.086 (r 2 = 0.983) and for all specimens as W= 0.007 L 3.079 (r 2 = 0.983). Regression coefficient b in males, females and all individuals was more than 3 and based on Pauly’s test, they were significantly different from the expected 3 ( p 0.05), indicating a positive allometric growth pattern in all the specimens. It was determined that mean condition factor (CF) was 0.927 ± 0.009 for males and 0.932 ± 0.006 for females and not significantly different between the sexes. The minimum and maximum absolute fecundity were 604 and 6558 (3348.24 ± 200.52), respectively. There was significant correlation between fecundity and length and weight (r 2 0.5). The average egg diameter was 0.65 ± 0.01 (± SE) mm, the minimum and maximum egg diameter was 0.267 and 1.70 mm. The mean of gonadosomatic index (GSI) was 1.53 ± 0.103 in males and 4.37 ± 0.237 g in females. The GSI showed significant differences in different months in both males and females (p 0.05). The maximum value of GSI in females was observed in March and was significaly different from those of other months ( p 0.05). According to histological studies, five sexual maturity stages including: immature, in maturing, ripening, ripe and spent, were observed. According to GSI values, egg diameter and histological studies, it can be expressed that this species has a short spawning period during late March to early April. Our results indicated that the ovary in King nase is group synchronous. Key words : King Nase, Age, Histological, GSI, Fecundity, Bibi-Sayyedan River Semirom