Abstract Multiwalled carbon nanotubes have been synthesized by catalytic chemical vapor deposition of ethylene, using two different catalysts in order to obtain nanotubes with average diameters of 24 and 58 nm, and different lengths. Polyamide 6 (PA6) was reinforced by melt‐mixing in an extruder with 2 wt% functionalized and unfunctionalized carbon nanotubes (CNTs). The nanocomposites were characterized by thermogravimetric analysis, differential scanning calorimetry, and sliding wear determination (SWD). SWD results show a reduction of the residual depth in a scratch test. The reduction is larger in the presence of unfunctionalized CNTs. A further reduction is obtained when decreasing the nanotubes diameter, an effect which is related to larger interfaces between the nanotube and polymeric matrix. CNTs enhance the crystallinity of the polymer and act as nucleation sites, thus increasing the crystallization temperature in respect to neat PA6. The addition of carbon nanotubes helps the formation of PA6 in α‐crystalline form, which is thermodynamically more stable that the γ‐form. CNTs also can act as heterogeneous nucleation sites, hampering the movement of the polymer chains and increasing the crystallization temperature. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers