European catfish Silurus glanis L., 1758, belonging to family Siluridae and order Siluriformes, which are found in water bodies of north of Iran and one of their distribution points is Anzali lagoon. Totally 154 specimens from Oct.2005 to Sep.2006 were study for age, body size, gonad weight, liver weight in Anzali lagoon. In this study reproduction cycle, type, season and optimum temperature of spawning period was studied by Gonado-Somatic Index (GSI), Hepato-Somatic Index (HSI) and Condition Factor (CF) in 2006 year. The maximum mean of Gonado-Somatic Index (GSI) reach to 7.82 ± 3.04 (%) in May and June, then GSI value started to decrease from July to September in the end of spawning period. From second stages of maturation GSI Value significantly increased and showed maximum amount in the 5th stages of maturity, then decreased in the 6th stages. Result of this study shows that development of sexual maturity of gonad reached maximum values in April and May and in this period water temperature was 19.33 - 22.73 ºС. Variations in GSI values show that reproduction has annual cycle which begins in September and ends in June of next year. The rate of HSI had fluctuation, during the study period .The maximum value (1.95±0.22%) of it’s was in November and the minimum HSI (0.79 ± 0.12%) in July. The Condition Factor also changed during reproduction cycle and it was maximum in September (0.7±0.18%) and minimum in June. Concerning these investigation Silurus glanis is has a Prolonged Spawning period extending from the April to half of June.