Diamond nanoparticles possess numerous industrial applications, especially in the areas of heat transfer, catalysis, advanced materials, medical, electronics, and sensors. The rheological behavior of diamond-based nanosuspensions is experimentally investigated. The dispersions of diamond nanoparticles in highly refined thermal oil (THO) with 70�99 wt hydrocarbons (C15-C50) are prepared at varying particle loadings using two-step technique. Different surface characterizations are performed for the nanoparticles. The investigation is carried out for considerably high diamond nanoparticle loadings in THO, i.e., 0.1 wt to 1 wt. A combination of ultrasonication and stabilizer-addition method is applied to obtain ultrastability (four months) for factual applications of nanofluids. The viscosity of diamond-THO nanofluids is measured at varying temperatures and shear rates in the range of 298-338 K and 500�2000s�1, respectively. The obtained results affirm the non-Newtonian and shear thinning character of diamond-THO nanofluids. The shear-thinning behavior is quantified using Ostwald-de-Waele relationship. A maximum increase of 21 in the viscosity is found for 1 wt diamond nanosuspension at 298 K. The experimental data of viscosities is compared with the VFT (Vogel-Fulcher-Tammann) eq. A generalized multivariable correlation for the viscosity of diamond-THO nanofluid is presented as a function of temperature and nanoparticle loading. © 2018 Elsevier B.V.