A fuel cetane determination device for an engine is disclosed. In one embodiment, the device comprises a controller that is arranged and configured to determine a rapidness of combustion based on a parameter relating to a combustion state of an engine. The controller is further arranged and configured to determine the cetane value of the fuel in use by the engine based on the rapidness of combustion from a correlation between the rapidness of combustion and the cetane value when the engine is in a first operating condition. The controller is further arranged and configured to determine the cetane value of the fuel in use by the engine based on the rapidness of combustion from a correlation between the rapidness of combustion and the cetane value when the engine is in a second operating condition. The correlation for the second operating condition has a different characteristic tendency from the correlation for the first operating condition. A method is also disclosed.