A number of methods have been developed to evaluate communication protocol performance based on formal specifications. Most of them assume the specifications are given in the form of Extended Finite State Machines (EFSM). The service times of the transitions are key parameters in the performance analysis and are often assumed to be known apriori. This paper presents a methodology to estimate transition service times by testing the protocol implementation as a black box. An approach to deriving test sequences for transition testing is also proposed.