This paper focuses on the determination of the heat flow density (HFD) and interfacial heat transfer coefficient (IHTC) during the high pressure die casting (HPDC) process of AM50 alloy. A specially designed “step shape” casting is used during the die casting experiment. Based on the temperature measurements inside the die, HFD and IHTC are successfully determined. Calculation results indicate that HFD and IHTC at the metal-die interface increases sharply right after the fast injection process until approaching their maximum values, and after that their values decrease to a much lower level until the dies are opened. Casting thickness has a great influence on both of the HFD and IHTC. Process parameters, such as the intensification pressure, the piston velocity, have little influences on HFD while on the other hand the die temperature has a great influence on the HFD. The IHTC seems to be independent upon all those process parameters so the IHTC peak values maintain at a particular level when the casting thickness is fixed.