Protocol verification is an activity to assure the correctness of a communication protocols. Global state reachability analysis is one of the most straightforward and easily automated protocol verification methods. In order to improve the efficiency of global state reachability analysis, many global state reduction techniques and various verification methods, e.g., the incremental protocol verification method, have been proposed in the past decade. Most of these techniques and methods are based on the state transition model, e.g., the Communicating Finite State Machine (CFSM) model. Nevertheless, two of the Formal Description Techniques (FDTs) for communication protocols, i.e., ISO's ESTELLE and CCITT's SDL, are based on the extended state transition model, e.g., the Extended Communicating Finite State Machine (ECFSM) model. Therefore, most of the previously proposed reduction techniques and verification methods cannot be directly applied to verify protocols specified in ESTELLE or SDL. This paper proposes a new incremental protocol verification method for the ECFSM model. The new method is based on the concept of dead and live variables proposed in Chu and Liu's ECFSM reduction technique . By analyzing the dead and live variables, global states which have the same values for live variables and different values for dead variables can be treated as equivalent. Dead variable and live variable sets may be changed when the protocol is modified. Therefore, equivalent property of global states may be changed accordingly. As a result, the change of dead and live variable sets needs to be considered in the incremental verification process. Based on the new method, incremental verification process can be directly applied in ESTELLE or SDL verification tools.