Multiwalled carbon nanotubes (MWNTs)/nylon-6 (PA6) nanocomposites with different MWNTs loadings have been prepared by the simple melt-compounding approach. A fine and homogeneous dispersion of MWNTs throughout PA6 matrix is observed by transmission electron microscopy. Scanning electron microscopy observation on the fracture surfaces of the composites shows not only a uniform dispersion of MWNTs but also a strong interfacial adhesion with the matrix, as evidenced by the presence of many broken but strongly embedded carbon nanotubes (CNTs) in the matrix and by the absence of debonding of CNTs from the matrix. Beadlike morphology is also observed along the stretched CNTs and their bundles, probably indicating the anchoring locations of the CNTs defects (within the beads) along the tubes where the PA6 matrix has strong interfacial interactions with the CNTs, thus being favorable to stress transfer from polymer to CNTs. Mechanical testing (by tensile and nanoindentation tests as well as dynamic mechanical analysis) shows that, compared with neat PA6, the elastic modulus and the yield strength of the composite are greatly improved by about 214% and 162%, respectively, with incorporating only 2 wt % MWNTs. In addition, a unique crystallization and melting behavior of MWNTs/ PA6 composites are observed and discussed by combining differential scanning calorimetry and X-ray diffraction; that is, only the alpha-form crystals are observed in MWNTs/PA6 composites, which is quite different from the case observed in PA6/clay nanocomposites.