Diesel engines are the predominant power source in trucking industry. Heavy duty trucks move more than 70% of all goods transported around the United States. The atmospheric conditions vary with altitude but are vital to diesel engine performance, efficiency, and emissions. Existing studies reported reduced thermal efficiency and increased emissions when truck engines were operated at high altitude. As the heat loss is a key parameter related to engine efficiency, the goal of this paper was to investigate the altitude impacts on in-cylinder heat transfer characteristics. A single cylinder four-stroke heavy duty diesel engine was performed at constant speed and load but different intake pressure to simulate the varying atmospheric conditions at different altitude. The engine raised the amount of diesel mass injected to the cylinder per cycle to maintain the identical power output under decreased atmospheric pressure and to compensate the combustion deterioration happened inside the cylinder. The experimental results indicated a higher bulk temperature at high altitude due to a smaller amount of mixture mass trapped inside the cylinder. Such a larger temperature difference between the hot products and the cold walls increased in-cylinder heat transfer to the coolant, especially during the combustion period. Specifically, a rise in 2000m altitude resulted in up to ∼2% increment in heat loss to the atmosphere per fired cycle. As a result, applying thermal coating to improve fuel economy is more necessary in high altitude states, such as Colorado and Wyoming.