Finite State Machines (FSMs) are widely used when deriving tests for components of discrete event systems. In general, the specification FSM can be nondeterministic and in this case, a test suite with the guaranteed fault coverage is derived with respect to the reduction relation. However, when deriving such tests for nondeterministic FSMs, the existing methods return rather long test suites which cannot be used for real systems. In order to shorten a test suite, the set of possible implementation FSMs can be reduced. We present an approach for deriving shorter test suites for nondeterministic FSMs with respect to the reduction relation via refining the specification FSM.