This paper aims to study the effects of sizing on properties of differently sized carbon fiber (CF) reinforced poly(butylene terephthalate) (PBT) composites by comparing them to unsized CF reinforced composites. Contact angle analysis was used to evaluate the wettability of CFs and the work of adhesion between the sizing agent and PBT matrix. It was found that wettability of PU sizing material by PBT matrix was better than that of other sizing materials by PBT matrix. Tensile and dynamic mechanical analysis (DMA) tests were performed to investigate the effect of sizing agent type on mechanical and thermomechanical properties. According to tensile test results PHE and PU sized CF reinforced PBT composites gave higher tensile strength and modulus values than the others. DMA revealed that PU sizing material gave better adhesion strength than other sizing materials. It was found that electrical conductivity values of all composites are about 10(-2) S/cm. SEM analysis showed that PU sized CF surface covered a layer of PBT matrix in accordance with other test results. As a conclusion of all results, it can be suggested that PU is a proper sizing material to be used for CF surface for PBT matrix. (C) 2015 Elsevier Ltd. All rights reserved.