Abstract: Molecular dynamics simulations of carbon nanotube/polyethylene composites have been implemented to calculate the structures, thermodynamical and mechanical properties, investigating their relationship with the filling rate and simulation temperature. The results show that carbon nanotube/polyethylene composites are in isotropic amorphous structures, with the polyethylene and carbon nanotubes combining together by intense van der Waals force. The periodicity of carbon atomic arrangement on carbon nanotube wall declines within the effects of polyethylene matrix, representing flexural and plicate deformations. Based on the energy consideration, the higher stability can be obtained with more carbon nanotube filling in composites. The carbon nanotube/polyethylene composites also exhibit higher isometric heat capacities, and negative thermal pressure coefficients with better temperature stability, which decrease explicitly with increased temperature and composite filling rate respectively, compared with polyethylene system. The mechanical properties of carbon nanotube/polyethylene composites present isotropic elastic constant tensors with substantially higher elastic modulus and Poisson ratio than those of polyethylene system, both of which decline with elevated temperature and reduced carbon filling simultaneously, indicating the extraordinary improvement in mechanical properties with the carbon nanotube filler.