The reproductive biology of bigeye tuna,Thunnus obesus,was investigated by sampling 1 042 fish caught in the tropical Atlantic Ocean during 2003 to 2006,1 313 fish caught in the westcentral tropical Indian Ocean during 2003 to 2005 and 391 fish caught in Eastern and Central Tropical Pacific Ocean during February through November 2006.The paper analyzed and compared the length at 50％ sexual maturity(L50)of bigeye tuna(Thunnus obesus)in the three oceanic regions using 5 models.The results showed that the observed minimum length at sexual maturity for female (male)bigeye tuna in the tropical Atlantic Ocean,female(male)bigeye tuna in the west-central tropical Indian Ocean,and female bigeye tuna in the east-central tropical Pacific Ocean to be around 104 cm(108 cm),110 cm(110 cm)and 94 cm,respectively.The similar results can be concluded using Lysack(1980),White(2001)and Bakhayokhos(1983)models.The maximum estimated L50 was derived from the model of Richards(1959)and the minimum is from Gempertzmodel.For the maturation rate(the rate that fishes were attained maturity),the maximum and minimum of female bigye tuna is from Richards(1959)and Lysacks(1980)model,respectively,male bigye tuna,however,is from Lysacks(1980)model and from Gempertzs model,respectively.Compared the 5 models,the minimum Akaikes information criterion value is from Lysacks(1980)and can be accepted as the best model for estimating L50 of bigeye tuna.