Viscosity plays a significant role in the utilization of bio‐oil as a liquid transportation fuel. This study investigated the influence of temperature on the viscosity of bio‐oils derived from white pine and from Norway spruce needles. The bio‐oils were produced by pyrolyzing the biomass samples in a fixed‐bed batch reactor at a temperature of 500 °C for 30 min. The viscosity of the bio‐oils was determined at a temperature range of 20–80 °C. The bio‐oil yield was estimated to be 36.32% and 28.84% for white pine and Norway spruce needles, respectively. The viscosity of bio‐oils derived from white pine and Norway spruce needles decreased from 177.67 to 54 cP and from 161.17 to 37.67 cP, respectively, as the temperature increased from 20 to 80 °C. Among the three models (Arrhenius, modified Williams–Landel–Ferry, and the power law) used to describe the temperature‐viscosity relationship of the bio‐oils, the Arrhenius model presented the best fit for the experimental data with activation energy of 16.80 and 20.79 kJ/mol–1 for bio‐oils derived from white pine and Norway spruce needles, respectively. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd