Longtail tuna (Thunnus tonggol) is one of the most economically important tuna species in Iranian waters of Persian Gulf and Oman Sea. Some population dynamics parameters of this species were investigated in coastal waters of the northern Persian Gulf and Oman Sea from January to December 2014. The fork length (FL) of all specimens ranged from 27 to 107 cm, and total weight (TW) was between 200 and 12,000 g. Among the length frequencies examined, specimens with fork length in the range of 62-65 cm were the dominant group. The equation of length-weight relationship was estimated as TW = 0.00003 9 FL , respectively. The exploitation rate (E) was calculated as 0.63 indicating that longtail tuna was fully exploited in the study region.