The flathead mullet (Mugil cephalus) is an important food fish species in the mullet family Mugilidae. The subject flathead mullet is widely distributed in tropical and temperate rivers and seas, and found abundantly in Myanmar. They are available throughout the year and of commercial value. In the present study the sex ratio and spawning condition of the fish were examined. A total of 344 specimens of Mugil cephalus were collected from a segment of the Bago River, 16 ° 49′ N to 16 ° 51′ N, and 96° 18′ E to 96 ° 20′ E. The sex ratio of Mugil cephalus was 1:1 during the study period. The highest GSI value of males was (1.90 ± 0.76) in April with the testes weight (1.08 ± 0.83) g and female was (15.68 ± 1.67) with the ovaries weight (92.50 ± 12.5) g in March. Based on the gonad weight, gonadosomatic index (GSI), hepatosomatic index (HIS), condition factor (k) value and histological study of gonads, the spawning season for M. cephalus was determined to be February to May.