DFM (Dynamic Flowgraph Methodolgy) is very suitable and powerful for DICS dynamic reliability analysis, which recommended by NASA and NRC. In this paper, advance of DFM are recalled at first, then its limitations are analyzed and discussed based on benchmark of DICS given by NRC. Improvements focused on treatment of uncertainty caused by discretization error and speeding up its 'prime implicants' derivation procedure are also proposed.