Abstract: The solution mixing was used to fabricate a series of carboxylic multi-walled carbon nanotubes (CMWNTs ) /poly ( adipic acid- hexamethylene diamine ) (PA66 ) composites. The thermal and crystallizing properties of the composites were characterized by the thermogravimetric analysis (TG), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarizering microscope ( POM). It is found that as the content of CMWNTs increases, the melting temperatures of the composites are almost unchanged, and the crystallinity and crystallizing temperature increase. CMWNTs act as a heterogeneous nucleation agent and do not affect the crystal form. POM analysis shows that addition of CMWNTs reduces the grain size of PA66. The interaction between CMWNTs and PA66 molecular chain is mainly Van Der Waals force and hydrogen bond. No evidence is given to prove the existence of chemical bonds between CMWNTs and PA66 molecular chains. The decomposition temperature of the composites rises slightly. The strong absorption of CMWNTs to the free radicals formed in the decomposition process decreases the rate of the composite decomposition.