Abstract: Several important functions (geometry important function, forward weight window, and adjoint weight window) of MCNP program were introduced in this paper. Geometry important function has some advantage of easy application, and it is widely used for nearly all models. Weight window can save substantial computer time when it is used judiciously. The mesh-based forward weight window can be used for almost all models. The adjoint weight window has almost the same advantage as forward weight window, but it is almost unavailable for complex geometry model because MCNP program can’t calculate the volume of some cells (such as repeated structure cell or some asymmetrical cell), which is indispensable for adjoint weight window generator. Two numerical examples were provided in this paper, and the computation efficiency of forward weight window is higher than that of the other two important functions.