Nanofluids have gained a lot of attention from scientists and companies in the last few decades due to their enhanced properties such as thermal conductivity. Carbon nanotubes are nanoparticles which are recognized for their outstanding thermal conductivity. For that reason, they are often being used as nanoparticles in nanofluids. Aim of this work was to prepare and investigate nanofluids of polyethylene glycol-200 (PEG- 200) with four types of multi-walled carbon nanotubes (MWCNT) in the concentration range of 0.1; 0.25; 0.5; 0.75; 1; 2.5; 5 and 10 wt.%. Nanofluids have been prepared by ultrasonic mixing of PEG and four types of MWCNT differentiating in length and radius. Two types of MWCNTs were pristine, and two were oxidized, containing carboxyl (COOH) functionalized group. The influence of length, width, functionalization and concentration of MWCNT on thermal conductivity and rheological properties of nanofluids was studied. Thermal conductivity and rheological properties of base fluid were also investigated. It was observed that nanofluids containing 10 wt.% of short and wide nanotubes have the biggest increase of thermal conductivity (133 %) in the reference to the base fluid. Nanofluids with types of MWCNT which contain carboxyl group have better heat transfer properties compared to the pristine. In the case of nanofluids with long and narrow oxidized CNT there was an increase in thermal conductivity of 112 %. Nanofluids in the concentration range 0.01-1 wt.% of MWCNT have Newtonian behavior and those with higher concentrations have non-Newtonian behavior. For nanofluids containing short and wide nanotubes shear thickening behavior is observed, and those containing long and narrow nanotubes have shear thinning behavior. By increasing the temperature and decreasing concentration of MWCNT in nanofluids, viscosity decreases.