The polyamide-based thermoplastic elastomers (Pebax$^{(R)}$) were melt compounded with multi-walled carbon nanotubes (MWNTs: 0.25~5 wt%) and the variation of rheological and physical properties with MWNT contents was investigated. The crystallization temperature ($T_c$) of the nanocomposites with 0.5 wt% MWNTs was most increased by ca. $8^{circ}C$, but it was decreased by further addition. In addition, the presence of MWNTs broadened the $T_c$ peak with increasing nanotube contents. In contrast, the melting behavior was little influenced by the presence of MWNTs for all compositions. The incorporation of MWNTs increased the complex viscosity with MWNT contents and the abrupt increase was observed from 1 wt%. In addition, lower Newtonian flow region became disappearing with increasing MWNT contents, exhibiting notable shear thinning behavior from 1 wt% loading. Storage modulus was increased with MWNT contents in a similar manner to viscosity. Casson plot demonstrated a non-zero positive intercept for all the samples. In particular, the abrupt increase of yield stress was observed from 1 wt% loading. In the Cole-Cole plot, the nanocomposites gave a deviated curve from pure Pebax and the slope was decreased with increasing MWNT contents. The relaxation time calculated from viscoelastic parameters was increased with nanotube contents, but the increasing extents were reduced with increasing frequency. From 2 wt% MWNTs, the electrical conductivity was observed, indicating that the electrical percolation existed between 1.5 and 2 wt%. At 0.25 wt% loading the tensile strength was slightly increased, but it was gradually decreased by further addition. The introduction of MWNTs increased the tensile modulus with nanotube contents. In addition, ductile properties were reduced with increasing MWNT contents, resulting in low toughness.