The main goal of this research is to study the tensile behavior of embedded short carbon nanotubes (CNTs) in a polymer matrix in presence of van der Waals (vdW) interaction as inter-phase region. A 3D finite element model of a unit cell consisting of capped carbon nanotubes, inter-phase and surrounding polymer is built. The unit cell is subjected to tensile load case to obtain longitudinal Young's modulus of the investigated cell. A parametric study is carried out to investigate the effect of CNT's length on reinforcement. It is observed that improvement in the Young's modulus of CNT-composite is negligible for lengths smaller than 100 nm and saturation takes place in larger lengths on the order of 10 lm. Furthermore, a comparison between results obtained for short carbon nanotubes and long carbon nanotube is presented. The efficient length of CNT in form of (10, 10) is obtained at the order of 10 lm. Finally, it was shown that direct use of micromechanics equations for short fibers will overestimate the stiffness. However, employing effective stiffness of equivalent fiber comprising of CNT and its inter-phase instead of high modulus of CNT will lead us to more appropriate results, which are in an acceptable agreement with conventional semi-empirical micromechanics equations.