Natural Gas (NG) is an alternative fuel, which has attracted a lot of attention recently. The higher H/C ratio, compared to gasoline, is capable of decreasing CO2 emissions throughout the entire engine map. Furthermore, the high knock resistance of NG can increase the efficiency at high engine loads. NG direct injection (DI) allows for fuel to be added after intake valve closing (IVC), during the intake stroke, resulting in an increase in power density compared to an injection before IVC. Steady-state tests were performed on a single-cylinder research engine equipped with gasoline port-fuel injection and NG DI to allow for in-cylinder blending of both fuels. Knock investigations were performed at two discrete compression ratios, 10.5 and 12.5. Operation conditions span mid-load, full-load and boosted operating conditions, depending on the knock response of the fuel blend. The two gasoline fuels have known values that characterize knock resistance which were used as a reference for the knock resistance of the NG/gasoline fuel blends. The spark timing was varied at different loads under stoichiometric conditions in order to study the knock response as well as the effects on performance and efficiency. Mathematical correlations were developed for characterizing the occurrence of knocking combustion by using the Livengood-Wu integral. As anticipated, results suggest that the knock resistance can be increased significantly by increasing the NG amount. Constraining an engine operation at maximum break torque timing, the load range could be increased by 14bar operating the engine with 75% NG compared to the engine operation with a low knock resistant gasoline at CR 12.5. The mathematical correlations show good agreement when compared to experimental data. However, it was found that the determination of the experimental knock onset location has significant influence on the agreement between experimental and mathematical knock onset location.