The performance of a composite material system is critically controlled by the interfacial characteristics of the reinforcement and the matrix material. Here we report a study on the interfacial characteristics of a carbon nanotube (CNT)-reinforced polystyrene (PS) composite system through molecular mechanics simulations and elasticity calculations. In the absence of atomic bonding between the reinforcement and the matrix material, it is found that the nonbond interactions consists of electrostatic and van der Waals interaction, deformation induced by these forces, as well as stress/deformation arising from mismatch in the coefficients of thermal expansion. All of these contribute to the interfacial stress transfer ability, the critical parameter controlling material performance. Results of a CNT pullout simulation suggests that the interfacial shear stress of the CNT–PS system is about 160 MPa, significantly higher than most carbon fiber reinforced polymer composite systems.