This study investigated the damage accumulation behaviors in carbon fiber reinforced nanocomposite laminates under tensile loading. The nanocomposite laminates used in this study were manufactured from prepregs consisting of traditional carbon fibers and epoxy resin filled with cup-stacked carbon nanotubes (CSCNTs). Thermo-mechanical properties of unidirectional carbon fiber reinforced nanocomposite laminates were evaluated, and cross-ply laminates were subjected to tension tests in order to observe the damage accumulation behaviors of matrix cracks. A clear retardation of matrix crack onset and accumulation was found in composite laminates with CSCNT compared to those without CSCNT. Fracture toughness associated with matrix cracking was evaluated based on the analytical model using the experimental results. It was suggested that the dispersion of CSCNT resulted in fracture toughness improvement and residual thermal strain decrease, which is considered to cause the retardation of matrix crack formation.