Abstract:Improves standard assessments of many commercially valuable fish species as Blue line snapper Lutjanus coeruleolineatus, is considered as an important commercial species harvested in the traditional fishery in the Sultanate of Oman. This will increase our knowledge in fish industry management. The present investigation of some biological aspects of Blue line snapper including male-female sex ratio, condition factor (Kn), gonado somatic index (GSI), length at first maturity for both sexes and developmental stages of ovary. Histological changes during the annual reproductive cycle and the stage of maturity of the ovary of Blue line snapper, L. coeruleolineatus are reported. Random collection of the species (449 males and 529 females) were taken monthly from Dhofar Governorate off the coast of the Arabian Sea at the period from March 2015 to February 2016. The results of this study reported male-female sex ratio was significantly different (P<0.05) from 1:1with dominate of females. The average monthly condition factor (Kn) values showed quite similar trend for both sexes with a slight higher values for females. The gonado somatic indices (GSI) data indicate that L. coeruleolineatus has prolonged breeding season from August to January with two peaks occurring in October and January. The length at first maturity was 28.1 ± 0.0 cm for males and 29.7 ± 0.0 cm for females. In this study five developmental stages of ovary of L. coeruleolineatus are recognized during development. Where GW is the wet gonad weight (g) and TW is the total fish weight (g).The length at first maturity (L m50 ), at which 50% of both sexes were calculated by using logistic equation considered by Froese and Binohlan (2000):Where P is the proportion of mature females or males in each 1-cm length class, L is the mid-class length, L m50 is length at first maturity and a is a constant. Non-linear least-squares fitting with Microsoft Excel Solver problem was used to obtain the best fit of L m50 and a.