Tribological aspects of bio-oils have to be investigated before they are used as industrial fluids. In this paper rheological (fluid deformation under shear) properties of four bio-oils are presented. A Couette type viscometer was used to evaluate flow behavior of the oils at seven discrete temperatures. Commercial coconut, canola, sunflower and corn oil were used in this study. For all the oil samples, viscosities were affected by the change of shear rate and temperature. Two rheological models, temperature dependence and shear rate dependence, were used in the study. The original power law gives contradict picture of flow curves and flow parameters. Thus, modified power law model was proposed. The modified model was fitted to experimental data to obtain the flow behavior index. Arrhenius type equation was fitted to experimental data to obtain the activation energy due to different temperatures and shear rates. Evaluation of rheological properties by modified power law model indicates that these bio-oils belong to the pseudo-plastic group.