In order to investigate improvement on mechanical properties brought by carbon nanofiber (CNF) in the carbon fiber reinforced polymer, a Matlab program to generate geometry models with random and periodic CNF distribution was developed. MSC.Patran and ABAQUS commercial code were used to build represent volume element (RVE). A damage criteria was introduced to different phases and interfaces in composites. It is reported that in axial and transverse direction of composites there is little improvement on ductility. In contrast, ultimate strength decreased by 15%-30%. However, when introducing initial crack in carbon fiber, the presence of CNF can prevent propagation of the crack and improve the axial ultimate stress by 25%.