An age structured production model (ASPM) was used in this study to assess the yellowfin tuna, Thunnus albacares, stock in the Indian Ocean. Sensitivity analysis was conducted to evaluate impacts of steepness parameter of the stock-recruit relationship and natural mortality rates of different age groups on the stock assessment. The dynamics of this stock was quantified. This study indicates that the value of steepness should be 0.6-0.8 for the stock supporting maximum sustainable yield (MSY). Outputs from ASPM were the closest to the current stock status if natural mortality recommended by the Inter-American Tropical Tuna Commission were used. Total biomass and spawning biomass decreased with an increase in fishing effort. However, the total biomass tended to be stable and maintained at 1959-2632 thousands tons with an average of 2210 thousand tons. Spawning biomass decreased to a level below 1000 thousand tons after 1994, dropped to a level that was unable to support MSY after 1997, and continue to decrease at present. Recruitment varied dramatically in the beginning of fishery, varied between 3258.36-6583.35 million individuals with an average of 4687.66 million individuals. The abundance of immature individuals maintained stable, but the abundance of mature individuals decreased greatly from 246.51 millions at the beginning to 19.02 million. Total fishing mortality estimated by ASPM had increased from the beginning of the fishery, then increased to 0.334-0.456 after 1991, and exceeded FMSY in 2003. The catch also exceeded MSY in 2003. Current catch is too high to be sustainable. MSY was 364 thousand tons from 2003 to 2005 with S/SMSY equal to 0.76 and Fall/FMSY equal to 1.39. The average catch (464 thousand tons) exceeded MSY (364 thousand tons) level from 2003 to 2006. We conclude that overfishing occurs in the yellowfin tuna fishery in the Indian Ocean.