The deformation and fracture of unidirectional carbon fiber reinforced plastics (CFRPs) at on-axis tension, i.e., during tensile deformation along the fiber direction, was observed directly by transmission electron microscopy. The microstructural dynamics in the separation of carbon fibers from an adhesive resin matrix and subsequent fiber extraction were observed using an in situ deformation system. After the fiber extraction, resin flakes were left on the side surfaces of the fibers. The structure and the number density of the flakes and the effects of sizing treatments on them were investigated. The sizing treatment caused the increase in the length of the residual resin flakes along the fracture direction by approximately 400% and the decrease in the number density of the flakes by approximately 40% in comparison with desizing cases. It was found that the sizing treatment brought about easy elongation of the resin around the interfaces between the fiber and resin matrices and the decrease in the interfacial bonding strength. This leads to the quantitative evaluation of the variation in the interfacial bonding strength between the fiber and resin and the ductility of the resin matrix around crack propagation regions in both sizing and unsizing CFRPs. A model of the microstructural fracture process during on-axis tensile deformation was proposed, which contributes to a fundamental of CFRPs developments.