Abstract: Telecommunication systems are typical complex systems. Services that independently operate normally will behave differently when simultaneously initiated with another service. This behavior is called feature interaction and is recognized to affect the dependability. This article proposes a method of dramatically reducing the computation time required for detecting feature interactions in telecommunication services. One of the knotty problems in defecting feature interactions at the specification design stage is terminal assignment. For the same service specifications, occurrence of feature interactions depends on how to assign real terminals to terminal variables in the specifications. Consequently, all terminals connected to the network have to be considered in order to detect all interactions. As a result, the number of combinations of terminal assignments is enormous. This causes huge expansion of computation time needed for detection of feature interactions. By considering equivalent states, the proposed method can reduce the number of terminal assignments to one 400th compared with that of the conventional method.