A molecular dynamics (MD) simulations was performed to study the thermodynamic behavior of vacancies in crystalline silicon. In the simulation, we adopt the Stillinger-Weber potential used commonly for silicon to describe the interaction between atoms. Two kinds of methods were employed to trace the jump of a vacancy, and its activation energy in the crystal was also calculated. The statistic explanation of the definition of vacancy jump proposed by Thomas was given. Besides, we find that most of vacancy jumps are performed passing through a metastable state.