In this paper, experimental evaluation was carried out on a 6.0 L heavy duty CNG engine which has been optimized for 18 percent hydrogen blended CNG (HCNG). Optimization test results shows that use of HCNG results in reduced CO, THC & CH4 emissions by 39, 25 & 25 percent respectively and increase in NOx by 32 percent vis-a-vis CNG. After optimization the engine was subjected to endurance test of 600 hours as per 15 mode engine simulated city driving cycle with HCNG. The performance & emission characteristics of the engine were analyzed after completion of every 100 hours as per European Transient Cycle (ETC). Test results indicate that there were no significant changes observed in engine power output over the complete endurance test of 600 hrs with HCNG. Specific fuel consumption (SFC) measurements were consistent at all the 15 modes of engine simulated city driving cycle. It was observed from the test results that over the period of endurance test of 600 hrs with HCNG, all the major exhaust emissions species such as CO, THC, CH4 & NOx emissions were found to be much lower than the Euro IV emission norms for the heavy duty engines and there were no significant changes in the CO2 values. Considering the engine performance, emission and functional durability analysis, it can be concluded that the existing natural gas engines can be optimized for 18 percent hydrogen blended CNG and this can be a good substitute as transport fuel, especially for natural gas engines.