The objective of the study is to characterize combustion and performance of a multi-cylinder turbocharged direct-injection (DI) diesel engine at altitude conditions according to the International Standard Atmosphere (ISA). Experiments were performed on the 6.6-liter turbocharged DI diesel engine, a model similar to that of the Army’s Joint Light Tactical Vehicle. The engine was installed in the US Army Research Laboratory Small Engine Altitude Research Facility. Outside air temperature (OAT) and outside air pressure were independently controlled to match the ISA-OAT at selected altitude conditions: sea level, 1524, 3048, and 4572 m. The test engine is equipped with a single-stage variable nozzle turbocharger and Bosch CRIN 3 common-rail injection system. Three load conditions (i.e., low, mid, and high) were selected at 1400 rpm to investigate combustion and performance of the engine using Jet Propellant-8 (JP-8) fuel. JP-8 (now F-24) is the primary Army fuel used in most air and ground platforms. An injection timing sweep was performed at each engine load and altitude condition to characterize engine combustion for varying altitude conditions. For all experiments, brake mean effective pressure (BMEP) was held constant at each injection timing and altitude condition. The results show that engine combustion and performance were significantly impacted by the altitudes above 3048 m. Peak heat release rate increased with increasing altitudes and steeply increased at the altitudes above 3048 m. Higher-altitude operation significantly increased equivalence ratio, leading to richer combustion at the same BMEP and higher exhaust gas temperatures. Low load presented completely different reaction paths from the mid to high loads. Indicated specific fuel consumption increased by up to 8.7% at the high load, and indicated thermal efficiency decreased by as much as 8% at the same load condition. The highest efficiency point at each load and altitude condition was observed at the highest temperature at fuel injection and the shortest ignition delay injection timing.