This study focuses on deposit formation in a diesel engine fueled with straight vegetable oil (SVO) and its effects on engine performance and exhaust emissions. First, two-dimensional thickness distributions of the carbon deposits on the cylinder head were measured by a laser displacement meter after continuous engine operation on gas oil blended with SVO at 25%. The obtained results show that the carbon deposit thickness rapidly increases with SVO-blended fuel and reaches a higher level than with just gas oil. Second, the effects of fuel injector deposits on engine performance and emissions were examined. A small diesel engine was continuously operated by alternating between SVO and gas oil. Gas oil was injected for 1 hour before and after 6 hours of SVO operation to prevent the accumulation of SVO deposits inside the nozzle holes, and the process was repeated. The experimental results of 32 hours of operation, which differed from predictions, show that the fuel consumption and unburned emissions worsen sooner with gas oil/SVO/gas oil than with just SVO due to a decrease in the injection rate caused by deposits in the nozzle holes.