Some biological traits of Killifish (Aphanius sophiae)of CheshmeAli Damghan such as age and growth frequencies, condition factor, von Bertalanfy growth parameter, Gonadosomatic index, fecundity and egg diameter were studied monthly from October 2004 to November 2005. During the study 535 specimens were sampled with a net which bears a mesh size of 1mm. Sex ratio of Cheshme Ali population was 1 :♂ 1.367♀.There was significant difference between male and female (χ2=12.877, p=0.000)). Length and age frequency studies showed that female fishes were longer and older than male fishes. Male fishes had higher condition factor than females (F=40.037, p=0.000). Reproduction cycles of these fishes were estimated to be about 6 month and the peak of GonadoSomatic Index of females was in May. Eggs diameters were from 0.53 to 2.73 mm.