The molecular dynamics method is used in this paper to investigate the effect of vacancy defects on the elastic properties of armchair and zigzag singlewalled carbon nanotubes （SWCNTs）.The results show that the Young's moduli of armchair （5,5）, （10,10） and zigzag （9,0）, （18,0） singlewalled carbon nanotubes are 948, 901 GPa, and 804, 860 GPa,respectively. The armchair and the zigzag SWCNTs Young's moduli decrease and increase with the increase of the nanotubes' diameter, respectively. With the increase of the vacancy defect ratio, the Young's moduli will decrease. When the vacancy defect achieves a certain ratio, there appears a sudden slowdown in the curves of Young's modulus vs. vacancy defect ratio and a platform emerges. The influence of the bivacancy defects on the Young's moduli of the carbon nanotubes depends on the orientation of the defects. As the number of atoms between the two singlevacancy defects increases, when the defects are located in the direction of the axis, the Young's modulus decreases to certain value and fluctuates around it; but when the defects are located in the direction of the circumference, the Young's modulus decreases first, then ascends and finally tends to a constant, and the Young's moduli will decrease a little with increasing of the distance between the two singlevacancies. The reasons are analyzed by considering the features of the bond σ and bond π between the molecules and the theory of the shortrange coupling of the electron clouds between two defects as well as the theory of 51DB defect formation in the vacancy defects.