Tensile deformation behaviors and the Poisson's ratio of single-walled carbon nanotubes (SWCNTs) are numerically studied, using the molecular dynamics (MD) method. Effects of several structural features of crystal cells of SWCNTs, i.e., the size, chirality and strain, on their mechanical properties are analyzed systematically. The simulations indicate that Armchair SWCNTs (8, 8)-(22, 22) and Zigzag SWCNTs (9,0)-(29,0) can be stretched by 35%-38% and 20%-27% without sign of plasticity, respectively. The Young's modulus of SWCNTs under tension ranges from 960 GPa to 750 GPa as their radii increase. The Young's modulus of zigzag SWCNTs is higher than that of armchair SWCNTs. Additionally, three SWCNTs (9,9),(12,6) and (16,0) are investigated to obtain their Poisson's ratio under tensile and compressive loading. The results show that the Poisson's ratio of nanotubes decreases generally as the strain increases. Under the same tensile strain, the Poisson's ratio decreases as the chiral angles of SWCNTs decrease, while their Poisson's ratios increase under the same compressive strain.