The ignitability of an air/fuel mixture by a spark plug is crucial for spark ignition engine performance, especially at idle and lean fuel conditions. It has been demonstrated that electrode geometry plays an important role in mixture ignitability. Typical industry practice for comparison of a mixture's ignitability by a spark plug is measured by the coefficient of variance (COV) of the indicated mean effective pressure (IMEP). However, the COV of IMEP is influenced by many other factors. Often the COV of IMEP fails to identify the influence of different electrode geometries on mixture ignitability, which hinders the determination of an optimal electrode design.This paper makes use of the 2% mass burn duration and location (when the spark timing is known) to gage mixture ignitability by a spark plug. Using this parameter, results obtained are consistent with the COV of IMEP. The differences between electrode designs are more clearly shown in the difference of 2% mass burn duration than the difference of COV of IMEP. Based on the 2% mass burned duration and location, the ignitability of a mixture by spark plugs with different electrode geometries are compared. By making use of this parameter, we are able to identify improved electrode geometry for future spark plug design.