This paper focuses on comparing the mass fraction burned obtained by pressure sensing and ion sensing through a conventional spark plug. The mass fraction burned curves obtained from both methods at different engine operating conditions are compared. In addition, the 0-10%; 50%, and 90% mass fraction burn durations are also compared.Traditionally, the mass fraction burned for a spark ignition engine is obtained through the measurement of pressure signal and the well known Rassweiler-Withrow method, which requires an expensive pressure sensor and some rather complex instrumentation. Ion sensing through a spark plug is very cost effective and reveals much information about the combustion process in the cylinder. Instead of making assumptions to attain the mass fraction burned, ion sensing by a spark plug allows us to obtain the mass fraction burned information through the Wiebe function constructed by several critical events shown in the ion signal during the combustion.The results demonstrate that the mass fraction burned attained by spark plug ion sensing offers an exciting alternative for mass fraction burned measurement and may be used as a tool to further estimate pressure in a cylinder.