Biology aspects of cobia (Rachycentron canadum) were investigated from October 2005 to December 2006 in Northern waters of Persian Gulf (Hormozgan province).The reproduction cycle, sexual maturity, sex ratio, Fecundity, Lm50, feeding, length frequency, growth parameters and mortality of cobia were studied from total of 509 specimens ranged from 19 to 159 fork length. Gonadosomatic index peaked during spring and summer with main peak in June. Spontaneous spawning occurs around the year with peak in June. The overall female to male ratio was significantly 1 : 1.49 (P<0.05). Batch fecundity were estimated 1684954±118990 in 15 females. Relationship between total length and fecundity were calculated F=1.3717TL 2.9567 (r^2= 0.82). Feeding studies indicated that the bony fishes were main food of cobia (76%) and followed by crustaceans (25%) and mollusks (11%). Rays were least food items (22%). Maximum and Minimum GaSI were observed in March (33%) and August (0.07%) respectively. Females reached 50% sexual maturity at 81.25 cm TL. The total length -weight relationship was W=0.0042L3.1162 (r^2=0.9852). Thetotal length-fork length relationship was TL= 1.1561FL-2.533 (r^2= 0.9933). Growth parameters K, L, and t_0 were calculated by von bertalanfy growth equation: 0.11 (year1), 168.65 cm, 3.49 and -0.97 respectively. The instantons rate of total mortality (z), natural mortality (M) and fishing mortality estimated by catch curve analysis were 0.30, 0.25, 0.14 (year1). Exploitation rate and Tmax were 0.36 and 28 year respectively.