Nanoparticle colloidal suspensions, referred to as “nanofluids”, have potential application in many heat transfer areas because of their intriguing properties such as considerable increase in thermal conductivity, long-term stability, and prevention of clogging in micro-channels. Here we prepared surfactant-free, homogeneous, and stable nanofluids by suspending well dispersible multi-walled carbon nanotubes (CNTs) treated using concentrated acids into ethylene glycol base fluid. The CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. The effects of the straightness ratio, aspect ratio, and aggregation of the CNTs on the thermal conductivity of CNT nanofluids have been investigated. Our findings suggest that the thermal conductivity enhancement could be adjusted by tailoring the suspended CNTs in the nanofluids. Nanofluids containing relatively straight CNTs with an appropriate length distribution have optimized thermal conductivity.