Ignition delay time is key to any hydrocarbon combustion process. In that sense, this parameter has to be known accurately, and especially for internal combustion engine applications. Combustion timing is one of the most important factors influencing overall engine performances like power output, combustion efficiency, emissions, in-cylinder peak pressure, etc. In the case of low temperature combustion (LTC) mode (e.g. HCCI mode), this parameter is controlled by chemical kinetics. In this paper, an ignition delay time model including 7 direct reactions and 13 species coupled with a temperature criterion is described. This mechanism has been obtained from the previous 26-step n-heptane reduced mechanism, focusing on the low temperature region which is the most important phase during the two stage combustion process. The complete model works with 7 reactions until the critical temperature is reached, leading to the detection of the ignition delay time value. The resulting ignition delay times obtained with the 7-step model have been compared to those of a calculation using a full kinetics of n-heptane developed by Lawrence Livermore National Laboratory. This comparison has pointed out that the model reproduces the ignition delay time with a good accuracy with differences smaller than 2 CAD.