Pre-inflammation mixture formation during ignition delay period plays as a core factor of diesel combustion. The goal of this study was to get an in depth understanding about the effects of fuel-air mixing process during the period of ignition delay and initial heat release in the combustion process that strongly influence exhaust emissions. This study explored the ignition delay, combustion process and emissions in a high ambient temperature of diesel combustion. The Rapid Compression Machine (RCM) imitate real phenomena in a combustion chamber. This analysis used blending ratio of 15%, 10%, and 5% mixing of palm oil methyl ester with standard diesel fuel in B5, B10, and B15 diesel engines and standard diesel itself. Injection pressure ranged from 140 MPa to 80 MPa, meanwhile RCM atmospheric temperature ranged from 950 K to 750 K. From the observation of the data results, when the ambient temperature rise, the ignition delay would decrease for all of the fuel tested. The initial heat release becomes more prolonged to lower ambient temperatures, and its peak was observed to rise gradually and shorten the delay of ignition. High ambient temperature effects showed a reduction in emissions, particularly HC, CO and CO2.