In order to determine the order of the BAW ladder filter, the relationships among the outofband rejection of the filter and the capacitance ratio of the parallel/series FBAR as well as the order N of the filter are studied by using simulation method. The BAW ladder filters with one to six orders has been built based on the Mason model of FBAR. The value of capacitance ratio Cps of parallel/series FBAR is from 1 to 6. The BAW ladder filters with 1~6 orders are simulated and the left outofband rejection of the filter is taken for statistics and drawing curves. In addition, we use the same simulation method to simulate the filter by changing the orders (N = 1.5, ... 5.5) when the orders of the BAW ladder filter are including the halforder. The simulation results show that the outofband rejection the basically equal to the order of the filter when Cps is constant, while the outofband rejection will increase with the increase of Cps when the orders of the filter are constant. When optimizing the design, the area ratio of the parallel/series FBAR (ie, Cps) is generally set at 1 to 4. In this range, the outofband rejection of will increase by about 10 dB when the order of the filter increases one. When the order of the filter is increased by half order, the outofband rejection is increased by about half of the whole order, that is 5 dB, while the insertion loss in the passband remains substantially unchanged. Therefore, when designing the filter, the order of the filter can be initially determined according to the outofband rejection in the design specifications.