Fast pyrolysis offers a flexible and attractive way of converting biomass into a high energy density liquid, which can be stored and easily transported and handled for the production of heat, power, fuels or chemicals. In this work, several tests were carried out in order to determinate the rheological behaviour of a bio-oil obtained from the fast pyrolysis of wood chips, alone and mixed with char at different concentration. The effect of temperature and the addition of methanol on the viscosity of the bio-oil variation, versus the shear rate, have been studied and modelled. The results obtained showed clearly that these bio-oils are pseudoplastic fluids which can be described using the Ostwald de Waele model. The addition of char to the bio-oil increases the apparent viscosity and accentuates the non-Newtonian behaviour of the slurry with the increase of the solid concentration. However, the elevation of temperature and the addition of methanol decrease drastically the bio-oil viscosity. According to the non Newtonian behaviour of the bio-oil, and if we want to use the viscosity as a propriety for the stability determination, it's necessary to determinate first the rheological behaviour of the fluid and then, if it is a non-Newtonian, we have to be sure that the viscosity is measured at the same shear rate for comparison.