Cetane number or CN is a measurement of the combustion quality of diesel fuel during compression ignition. It is a significant expression of the quality of a diesel fuel. A number of other measurements determine overall diesel fuel quality - these other measures of diesel fuel quality include density, lubricity, cold-flow properties, and sulfur content.

Cetane number or CN is a measure of a fuel's ignition delay; the time period between the start of injection and the first identifiable pressure increase during combustion of the fuel. In a particular diesel engine, higher cetane fuels will have shorter ignition delay periods than lower cetane fuels. Cetane numbers are only used for the relatively light distillate diesel oils. For heavy (residual) fuel oil two other scales are used CCAI and CII. In short, the higher the Cetane number the more easily the fuel will combust in a compression setting (such as a diesel engine).

Generally, diesel engines operate well with a CN from 40 to 55. Fuels with higher cetane number have shorter ignition delays, providing more time for the fuel combustion process to be completed. Hence, higher speed diesel engines operate more effectively with higher cetane number fuels. There is no performance or emission advantage when the CN is raised past approximately 55; after this point, the fuel's performance hits a plateau.