Molecular dynamics (MD) simulation is performed to study the vacancy-interstitial annihilation in crystalline silicon. We choose the Stillinger-Weber potential , which is commonly used for silicon, to describe the interaction between atoms. The system is relaxed under 300K and 1400K respectively. We have found that is the preferred recombination direction and propose the presence of an energy barrier in the direction. From the calculated value of energy barrier alon g we give a reasonable explanation for the difference between Tang's and Z awadzki's data.