A control system for an internal combustion engine, which is capable of estimating a property of fuel accurately even when the engine is in an operating condition other than idling. An in-cylinder pressure sensor detects an amount of change in pressure in a cylinder #1 of the engine as a pressure change amount. An ECU calculates an amount of heat released in the cylinder #1 as a heat release amount, according to the pressure change amount. A vehicle speed sensor, a crank angle sensor, and an accelerator pedal opening sensor detect load on the engine. The ECU estimates a cetane number of fuel based on the calculated heat release amount and the detected load on the engine.