Bisimulation is adopted in the π-calculus as the criterion to identify processes. For finite-state processes, bisimulation equivalence is decidable. In this paper, an optimization technique for a bisimulation checking algorithm is presented. It works by instantiating input names only to those free names which are used in subsequent matches. The technique can significantly reduce the required time and space, as illustrated by several benchmark examples. The correctness of the optimization is also proven.