Disclosed herein is a method of estimating the start of combustion (SOC) in a diesel engine using a difference between an in-cylinder combustion pressure and a motoring pressure so as to effectively control the combustion in a combustion chamber. The method of the present invention features that it comprises the steps of: detecting a DP value defined as a difference pressure between an in-cylinder combustion pressure and a motoring pressure; detecting a crank angle (CADPxx) at a point where the detected DP value equals a specific pressure; defining an angle difference between the start of combustion (SOC) and the crank angle (CADPxx) as a detected delay angle (ΔCADD) and determining the detected delay angle (ΔCADD) through an experiment of measuring the start of combustion (SOC) and the crank angle (CADPxx); and detecting the SOC using the following Equation: SOC=CADPxx−ΔCADD, whereby the SOC can be estimated simply and accurately, and that the detected delay angle (ΔCADD) is defined as a simple function for the engine speed.