Abstarct: In this study a new multiscale model for determining nonlinear behavior of carbon nanotube/polymer composite is proposed. Carbon nanotube (CNT) was modeled at the atomistic level baxsed on modified molecular structural mechanics using 3D beam elements of general section. The beam element of general section distinguishes the in-plane and out-of-plane bending stiffnesses, so that in this case the beam section geometry and its material descxriptions are combined. The modified Morse interatomic potential was used for descxription of atomic interactions in the CNTs. Some of armchair CNTs together with their equivalent zigzag CNTs in diameter, were subjected to tensile, torsional, and bending loading conditions to extract their mechanical properties from these loading conditions. The Young's modulus and the Poisson's ratio, the shear modulus, and the effective Young's modulus in bending of the CNTs were calculated from tensile, torsional, and bending loading conditions respectively. The obtained results were compared with the results existing in the literature, which showed good agreement. The polymer was taken into account as a continuum medium, and its 3D finite element model (FEM) was constructed using solid elements by considering its elastoplastic behavior. Using multiscale modeling, the representative volume element (RVE) was built through embedding the CNT within the polymer's model. The Interface between CNT and its surrounding polymer was considered as van der Waals forces defined by Lennard-Jones relation, which were modeled using nonlinear connector elements in Abaqus code. The RVE was subjected to tensile, torsional and bending loading conditions to extract its mechanical properties. The Young's modulus, the shear modulus, and the effective Young's modulus in bending of nanocomposite were calculated from tensile, torsional, and bending loading conditions respectively, and their variation with different volume fractions of CNT were investigated. The stress-strain curves resulting from tensile, torsional and bending loading conditions were reported. The obtained results indicated a magnificent improvement in the mechanical properties of the nanocomposite with respect to the pure polymer.