The objective of this research was to determine the relationship between engine performance and emissions of cottonseed oil biodiesel used in a 14.2 kW diesel engine. When using cottonseed oil biodiesel blends, CO, total hydrocarbon (THC), NO_x, and SO₂ emissions decreased as compared to petroleum diesel.  Carbon dioxide emissions had no definitive trend in relation to cottonseed oil biodiesel blends. Carbon monoxide emissions increased by an average 15% using B5 and by an average of 19% using B100.  Hydrocarbon emissions decreased by 14% using B5 and by 26% using B100.  Nitrogen oxide emissions decreased by four percent with B5, five percent with B20, and 14% with B100.  Sulfur dioxide emissions decreased by an average of 86% using B100, and by 94% using B50 blended with ultra-low sulfur diesel.  The difference between peak output power when using biodiesel and when using diesel was insignificant in blends less than B40.  Peak brake power when using B100 was about five percent lower than for diesel fuel.  Pure cottonseed oil biodiesel achieved and maintained a peak corrected brake power of 13.1 kW at speeds of 2990, 2875, and 2800 rpm at loads of 41.3, 42.7, and 43.8 N-m.  Using B5 produced a peak power of 13.6 kW at 2990 rpm and 43.9 N-m and at 2800 rpm and 46.7 N-m, while using B20 produced a peak power of 13.4 kW at 2990 rpm and 43.7 N-m.  Brake-specific fuel consumption at peak load and torque when using B100 was 1238 g/kW-h.  Brake-specific fuel consumption at peak power and loads using B5 and B20 were 1276 and 1155 g/kW-h.