A new criterion for the determination of the start of combustion (SOC) from the diesel engine in-cylinder pressure diagram was developed. It is defined asthe maximum of the third-order derivative of the cylinder pressure with respect to the crank angle. This criterion declares SOC more precisely than other previously published criteria based on pressure diagnostics. This fact was proven analytically and was discernable from the analysis of the experimental data. Besides its accuracy it is also robust enough to allow automatic evaluation of the SOC during processing of the pressure data for a large number of cycles. By applying the first law of thermodynamics analysis to the engine cylinder it was discovered that the third-order derivative of the in-cylinder pressure with respect to the crank angle is the most suitable criterion for determination of the SOC from the in-cylinder pressure diagram. Subsequently, the criterion was validated through experimental data analysis of the in-cylinder pressure diagrams for various engine speeds and loads. In order to evaluate the rate of heat release (ROHR), which formed the base for the experimental validation, in-cylinder pressure diagrams were processed witha computer code based on the first law of thermodynamics. The cylinder pressure was measured with an advanced piezoelectric sensor at the resolution 0.1 deg CA. Top dead center was determined with the capacitive top dead centersensor. Due to the analytic foundation of the developed method and its validation through highly accurate experimental data it can be concluded that new criterion is credible for the determination of the SOC.