Rheological behavior, pressure drop, and skin friction coefficient of multiwalled carbon nanotubes (MWCNT)-oil nanofluid were investigated experimentally. Plain and microfin tubes were used as the test tubes under constant heat flux boundary conditions. Pure heat transfer oil and nanofluuids with particle weight concentrations of 0.05%, 0.1%, and 0.2% were utilized as the working fluuids. Shear stress, shear rate, and viscosity of base fluuid of all nanofluuids at different temperatures were measured and the effect of nanoparticle concentration on fluuid properties was investigated. Furthermore, effects of nanoparticles concentration, heat flux, natural convection, and inclination on pressure drop and skin friction coefficient were studied for Grashof number ranging from 10(3) to 10(4), Reynolds number ranging from 10 to 150, and particle weight concentrations up to 0.2%. Results showed that pressure drop and skin friction increased as inclination was increased from 0 to 90 degrees. Results also showed that the effect of addition of nanoparticles on pressure drop in a plain tube is more compared to the microfin one. At the end, a correlation was developed to calculate the skin friction coefficient in different inclinations and nanoparticle weight fractions.