With the purpose to get more fundamental understanding of the theological behavior of nanofluids as filling the research gap in the literatures, the rheological behavior of ethylene glycol-based nanofluids containing spherical-shaped SiC nanoparticles have been conducted using a HAAICE MARS III rheometer at different volume fractions and temperatures. The results showed that the nanofluids exhibited an interesting theological behavior of the viscosity curves. Attempts were made to explain this phenomenon in this paper and the theoretical analyses revealed that the theological behavior of the studied nanofluids had a strong relationship with shear rates and particle concentration. In addition, this work concluded that viscosity of the studied nanofluids increased with volume fractions but decreased with temperatures. In addition, the theoretical analyses also lead to a classification of nanofluids into dilute nanofluids and concentrated nanofluids on particle concentration. (C) 2015 Elsevier Ltd. All rights reserved.