An applicable method for fatigue life prediction in the interfacial normal plastic strain (INPS) range is proposed for unidirectional carbon fiber-reinforced thermoplastic (CFRTP) specimens made of PA6. The INPS is defined as the plastic strain of the resin at the midpoint of the segment, which corresponds to the shortest distance in the direction of separation between two carbon fibers. The Drucker-Prager yield criterion is used for the microscopic elasto-plastic analysis of the specimen, which clearly distinguishes the resin and carbon fibers. The INPS range successfully predicts the fatigue of a 90°unidirectional CFRTP specimen by utilizing the relationship between the plastic strain range and number of cycles to failure of a PA6 specimen. In comparison to the von Mises yield criterion, the Drucker-Prager yield criterion for the CFRTP is advantageous in that the isotropic deformation of the resin is strongly constrained by the fibers.