A protocol modeling method is proposed to establish the protocol's OPNET simulation model based on its ESTELLE formal description. We use processes tree in OPNET to implement the embedded module mechanism in ESTELLE, and convert ESTELLE module FSM (Finite State Machine) to OPNET process FSM. This conversion technique is illustrated by applying it to model a specific subset of the MIL-STD 188-220C protocol. Since ESTELLE is a kind of international standards about network protocol formal description language (FDL) proclaimed by ISO, our modeling method not only guarantees the concordance for each phase in protocol project, but also increases efficiency of protocol modeling.