The focus of this study is to investigate the effects of oxidative functionalized carbon nanotubes (f-CNTs) and aminosilanized carbon nanotubes (s-CNTs) on the mechanical and thermal properties of polyamide 6 nanocomposites. Oxidation of nanotube surfaces was conducted with sulfuric acid/nitric acid mixture and then aminosilanization was carried out with -aminopropyltriethoxysilane. Nanocomposites were compounded by melt mixing technique and shaped by injection molding. Scanning electron microscopy images revealed that f-CNTs and s-CNTs were dispersed more evenly due to increased interactions with the matrix. Tensile tests indicated that yield strength and Young's modulus of nanocomposites increased 20 and 23%, respectively, with the addition of only 1 wt% s-CNTs due to very efficient load transfer from the matrix to covalently bonded CNTs. Both dynamic mechanical analysis and thermogravimetric analysis showed that surface-modified CNTs improve all thermal properties due to decreased matrix mobility and physical barrier formation. For example, increases in the storage modulus values were as much as 25%, while the increase in the thermal degradation temperatures were as much as 5 degrees C in the specimens with only 1 wt% s-CNTs.