The investigations including the acid treatment to multi-walled carbon nano-tubes (MWNTs) and the synthesis of MWNTs/polyurethane composites via in situ polymerization were done. X-ray Photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and dynamic mechanical analysis (DMA) were utilized for evaluating the effects of acid-treated MWNTs on the properties and microstructure of the composites. The results indicated that carboxyl groups could be successfully introduced onto the surface of MWNTs by acid treatment. The dynamic storage modulus and glass transition temperature of composites increased with the existence of MWNTs. The improvement of polyurethane by acid-treated MWNTs performed better compared to raw MWNTs.