Concern over the change of atmospheric conditions at high altitudes prompted interests in the deteriorated efficiency and emissions from heavy-duty diesel engines. This study utilized a single-cylinder, four-stroke, direct injected diesel engine to experimentally investigate the altitude effects on the combustion characteristics. High altitude operations were simulated via reducing the intake pressure but maintaining constant engine speed and load. The results suggested a reduced in-cylinder pressure trace but an increased bulk cylinder temperature as the altitude rose. The combustion analysis indicated a slight longer ignition delay, raising and retarding the pressure rise rate and energy release rate in the premixed combustion process. A smaller excess air ratio contributed to the combustion deterioration, reflected from a retarded end of combustion, a longer combustion duration, a reduced thermal efficiency, and an increased level of incomplete combustion. However, the phasing and combustion profile were not significantly impacted, when the altitude was elevated from sea level to 2000 m, at least for the engine and conditions investigated in this study. Consequently, it is not necessary to adjust the engine electric control unit (ECU) when operated in the USA, considering that the mean elevations of most states are lower than 2000 m.