The effective site-selective interaction between the π-bonds in the aromatic rings of the polyaniline and the graphitic structure of multiwall carbon nanotubes would strongly facilitate the charge-transfer reaction between the two components. We adopted multiwall carbon nanotubes (MWNTs) with minimal defects as templates and facilely fabricated carbon nanotube−polyaniline nanocomposites with uniform core−shell structures by ultrasonic assisted in situ polymerization. By varying the ratio of aniline monomers and carbon nanotubes, the thickness of polyaniline layers can be effectively controlled. The results indicated that the presence of carbon nanotubes with minimized defects induced the formation of a more planar conformation of polyaniline even when a high weight percent of aniline was loaded. As a result, macroscopically, great improvements in the electrical and electrochemical properties of the resulting nanocomposites were observed.