Carbon fiber-reinforced plastics (CFRP) have been studied increasingly for the last decades. Recently, thermoplastic-based CFRPs such as polypropylene, polyamides, etc. are much interested particularly in automobiles industries. Chopped carbon fibers as reinforcement have been accommodated for compounding, extrusion and injection molding processes to produce the CFRPs with highly improved properties and performances, compared to neat plastics and glass fiber-reinforced plastics (GFRPs). However, uses of chopped carbon fibers have been limited to produce high-performance CFRPs due to their low aspect ratio and, as a result, low mechanical properties, compared to the CFRPs with long carbon fibers. Hence, long fiber reinforced thermoplastics (LFT) are of interest to many composite industries. Most of commercial carbon fibers are normally sized with epoxy of low concentration because carbon fibers are most widely used with epoxy resins in a variety of applications. The sizing material may be substantially altered on industrial purpose as well as on research purpose. It is desirable that the sizing material may be appropriately applied to the carbon fiber surfaces, considering of the type of the matrix resin to be used for making a composite material. It is because the interfacial adhesion between the carbon fibers and the polymer matrix strongly influences the resulting composite properties. Consequently, the objectives of the present study are to produce various types of carbon fiber/polyamide 6 composites reinforced with PAN-based (Toray, T700 grade) chopped carbon fibers that are sized with different sizing materials such as epoxy, polyamide, polyimide, polyurethane, and maleic acid by using extrusion and injection molding processes and also to investigate how the sizing materials influence the interfacial, mechanical, and thermal properties of chopped carbon fiber/polyamide 6 composites. The composite with unsized carbon fibers are used for comparison.