Atherinidae family is the only species of Atherinidae family in the Caspian Sea is (A.caspiai). The present research was aimed at studying the age and growth of sand smelt in southwest of the Caspian Sea. Three hundred sixty fish specimens were caught using a small-meshed seine, fixed in 4% formalin and studied during four seasons on a monthly basis. The total length and weight of fish were determined to be from 57 to 127 with a mean of 90.30 ± 15.47 mm and from 12.62 to 1.28 with an average of 5.25 ± 2.36(g), respectively. The population has a 1 to 5 year life cycle and Mann-Whitney test showed a significant difference between males and females (P = 0.00).The ratio of male to female was 1.03:1, which means there was no significant difference between male and female ratio in the population (X² = 0.10, p > 0.05).  Length-weight regression analysis were (W=0.00003 TL^2.689)(n=183) in males, (W=0.00001 TL^2.824)(n=177) in females, and (W=0.00002 TL^2.762)(n=360) in total population, respectively, and the growth pattern in this species was determined as negative allometric. The von Bertalanffy growth function were calculated in sand smelt males, females and the whole population as  Lt _male=93.39 [(1-e ^{-1.36 (t-0.00)}], Lt _female=113.56 [(1-e ^{-0.77(t-0.00)}]and Lt _total=103.75 [(1-e ^{-1.01(t-0.00)}], respectively.