Viscosity, the measurement of the internal flow resistance of a liquid, constitutes an intrinsic property of vegetable oils. It is of remarkable influence in the mechanism of atomization of the fuel spray, in other words, in the operation of the injection system. This property is also reflected in the combustion process, whose efficiency depends the maximum power developed by the engine. This work aims at assessing the rheological behavior of castor oil, castor oil biodiesel, and undegraded and degraded biodiesel at different exposure times and temperatures. Castor oil biodiesel presents viscosity higher than diesel oil, but this drawback can be corrected by means of blends of both components at different proportions. The viscosity data indicated that the heat treatment leads to a degradation of the samples accompanied by an increase of the viscosity, probably because of interactions with intermediary compounds. The degraded samples presented a pseudoplastic behavior, once the flow index, m, is smaller than 1.