Lack of complete formal specification is one of the major obstacles to the deployment of model checking. Coverage estimation addresses this issue by revealing the unverified part of the design according to the specified properties. In this paper we propose a new transition-based coverage metric to evaluate the completeness of properties for symbolic model checking. Our coverage metric pinpoints the transitions through which the values of signals are checked. An efficient symbolic algorithm is presented for computing the transition coverage for a subset of ACTL. Our coverage estimator has been applied to the model checking of a cache coherence protocol. We uncovered several coverage holes including one that eventually led to the discovery of a design bug.