High pressure fuel injection systems, such as common rail (CR) systems and electronically-controlled unit injector (EUI) systems, have been widely applied to modern heavy duty diesel engines. They are shown to be very effective for achieving high power density with high fuel efficiency and low exhaust gas emissions. However, the increased peak combustion pressure gives additional structural stress and thermal load to engine structure. Thus, proper material selection and thermal analysis of engine components are essential in order to meet the durability requirements of heavy-duty diesel engines adopting a high pressure injection system.In this paper, thermal analysis of a 12.9 ℓ diesel engine with an EUI system was studied. Temperatures were measured on a cylinder head, a piston and a cylinder liner. A specially designed linkage system was used to measure the piston temperatures. A radio-tracer technique was also used to verify the rotation of piston rings. Parametric studies were carried out to understand the effects of several combustion parameters and engine operating conditions on metal temperatures. The parameters were fuel injection timing, flow rate and spray cone angle of a fuel injection nozzle, and temperatures of coolant, engine oil and charged air. The geometries of piston rings and piston ring grooves, and the flow rate of piston cooling jet were also investigated on the piston temperatures. These thermal studies proposed to modify the design parameters of pistons to reduce thermal load on pistons.