In this study, the effects of tall oil biodiesel with cobalt (Co)-based additive on engine performance and exhaust emissions have been experimentally investigated. Co-based additive at the rate of 4, 8, and 12 mol/l was added to mixtures of 60% tall oil methyl ester and 40% diesel fuel (T60). The engine tests were performed at full load condition in a single-cylinder diesel engine. Biodiesel fuels had no noteworthy influence on engine torque and the power output values. Catalyst effects of Co-based additive improved specific fuel consumption values, CO emissions, and smoke emissions. On the other hand, higher NOx emissions were measured at low engine speed.