Fiber-reinforced plastic composites (FRP) are getting more and more attractive in growing industry sectors due to their combination of low density with high specific strength and stiffness. Since the introduction of thermoplastics in the processing of FRP, their use in the field of continuous in-situ pultrusion is becoming increasingly important. Due to the excellent impregnation of the fibers with the reactive monomer melt based on ε-caprolactam, in-situ pultrusion probably offers the greatest potential of high-quality pultrudates with a maximum fiber volume content. However, the polymerization to PA6 in the pultrusion die significantly influences the quality of the pultrudates. In turn, decisive influence on the polymerization should have the temperature of the fibers, which are pulled in the die. For this reason, a fiber preheating by means of hot air is developed, in order to reveal its influence. Mechanical and chemical analyzes of produced pultrudated flat profiles with glass fibers clearly show better mechanical properties when using fiber heating. They achieve significantly higher strength values and a much higher degree of polymerization. The results obtained, show that there is a direct correlation between the temperature of the glass fibers fed in the die and the quality of the pultrudates. Due to these advantageous results using the developed fiber preheating, the existing state of the art of in-situ pultrusion should be extended in reference to the equipment technique by a fiber preheating as a standard component.