Mass Rapid Transit (MRT) Jakarta is one of the new urban transportation in Greater Jakarta area which will be operated in early 2019. The development period has been started in 2010. A research has been conducted to determine its train specification, including train set and car number in 2010. Time revealed that the based data for that research, forecasted number of Jakarta population, is not fits the actual number. This research goal is to determine the train specification, which are the number of MRT Jakarta train set and number of car for each train set to reach its headway target by considering MRT Jakarta daily passenger target based on actual number of Jakarta population. The researcher uses ProModel 7.5 as the tool to simulate 12 optional policy. These 12 optional policies is made of combined three control variable, which are train set, car number, and headway. Researcher use the number of passenger compared to MRT Jakarta daily passenger target as the indicator to choose the best policy. Based on the result of this research, the best train specification policy that could reach the 5 minutes headway MRT Jakarta target is 7 train set and 6 cars for each set.