This present paper described an experimental study on the combustion and emission characteristics of a diesel engine at idle at different altitudes. Five altitudes ranging from 550m to up to 4500m were investigated. Combustion parameters including in-cylinder pressure and temperature, heat release, fuel mass burning and so forth, together with emission factors including CO, HC, NOx and PM were tested and analyzed. The result of on-board measurement manifested that in-cylinder pressure descended consistently with the rising of altitude, while both the maximum in-cylinder temperature and exhaust temperature ascended with the altitude. It was found that ignition delay was lengthened at higher altitude, but the combustion duration became shorter. The crank angle towards 90% fuel burnt has hardly changed with the variation of altitude. As for heat release, the difference of slopes observed at different altitudes was quite slight. However, the peak heat release increased with altitude in the cases where altitude is lower than 3300m, and shrinkage appeared at 4500m. The curvature of pressure rise has duplicated the tendency of heat release. Further study on each emission factor yielded that an increase in altitude correlated to a continuous increase in CO and PM emissions. Generally, HC emissions had a positive correlation with altitude despite the slight fluctuation at mid-altitudes. Similar to heat release, a consistent increase in NOx emissions corresponded to an increase in altitude till the final drop observed at 4500m.