Carbon fiber–reinforced thermoplastic composites are gaining increasing interest in various applications thanks to their combined properties of high specific stiffness, high specific strength, and superior toughness. Their mechanical properties are highly dependent on the carbon fiber content. In this study, the carbonization-in-nitrogen method (CIN) developed in previous work is used to measure the fiber content of carbon fiber thermoplastic composites. Three types of carbon fiber thermoplastic composite samples were prepared using hot-melt impregnation. The carbon fiber thermoplastic composite sample is carbonized in a nitrogen environment alongside a neat resin sample that is used for calibrating the resin carbonization percentage. A good agreement is achieved between the nominal carbon fiber content and the carbon fiber content measured using the CIN method. It is concluded that the CIN method is an accurate and efficient way to characterize the carbon fiber content for carbon fiber thermoplastic composites. This work completes the verification of the CIN method, which enables extended application to thermoplastic composites. Moreover, it has its unique merits on evaluating the carbon fiber content for high-temperature and solvent-resistant thermoplastic composites that would encounter challenges using other methods.