The influence of different molecular weight sizing agent on the performances of carbon fibres and carbon fibres composites were studied. Three different kinds of molecular weight sizing were used. Surface composition of the fibres modified with aqueous sizing and topographies of carbon fibres surface were characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM) and scanning electron microscope (SEM). The interlaminar shear strength and hygrothermal ageing test have been used to study the effect of fibres coatings on the adhesion of surface. The results of the study indicate that the molecular weight of sizing agent has an important influence to the carbon fibres and carbon fibres composites. The high and low molecular weight sizing agent decreased the interfacial shear strengths and hygrothermal ageing of carbon fibres composite. The moderate molecular weight of sizing agent showed an improvement of the interfacial adhesion and hygrothermal ageing. Crown Copyright (C) 2010 Published by Elsevier B.V. All rights reserved.