The viscosity of sisal residue bio-oil was evaluated using the variation of the operational conditions of the fast pyrolysis process in a fluidized bed. The bio-oil was produced in a pilot plant from tests that were designed using the technique of experimental design. In this work, the effect of the flow rate of the inert gas (N-2), the biomass mass flow rate, and the reaction temperature on the viscosity at the shear rates of 40 and 75 s(-1) was investigated. The lowest viscosity was obtained when the N-2 flow rate and the temperature were at their lowest values (8 N m(3)/h and 450 degrees C) and the biomass flow was at its highest value (1560 g/h). The low-viscosity bio-oil was used to investigate the influence of the shear rates at its most critical flow condition (2-26 s(-1)) at temperatures of 60-110 degrees C. It was found that at 60 degrees C, the bio-oil viscosity ranged from 2699 to 353 mPa.s when the shear rate increased from 2 to 26 s-1. The lower value of this viscosity (353 mPa.s) is equivalent to the higher value reported in the literature. An empirical model of viscosity versus temperature was determined and compared with existing models. This evaluation was performed at three shear rates (2 s(-1) 14 s(-1), and 26 s(-1)) and at temperatures ranging from 60 to 110 degrees C. The model proposed in this study showed deviations much smaller than those obtained with the application of the most used models.