This paper reports on an experimental study conducted to determine the effect on the knock limited operating map of a natural gas engine when propane is added to the fuel. The map involves engine parameters such as BMEP, spark timing, equivalence ratio, and propane fraction. The map shows that to maintain its design BMEP, the maximum and minimum equivalence ratios that the engine can operate with natural gas are 0.78 at a timing of 25 degrees BTDC and 0.73 at 20 degrees BTDC, respectively. However, when the propane percentage of the fuel is increased to 15% of the fuel by mass, the maximum and minimum equivalence ratios that the engine can operate are 0.75 and 0.70, respectively, which corresponds to spark timings of 22 and 20 degrees BTDC. The map demonstrates that knock is not a major constraint for typical natural gas. Spark timing retard is limited by the exhaust gas temperature and minimum equivalence ratio is limited by the BMEP requirement of the engine. However, when propane fraction of the natural gas was increased knock was the major constraint and the lean mixtures were able to achieve the BMEP requirements.