This research work was aimed to investigate the maximum possible utilization of hydrogen energy in CI engine with diesel fuel replacement on energy share basis in hydrogen diesel dual fuel (HDDF) mode using common rail direct injection diesel engine. In HDDF mode, experiments were performed at two load conditions such as 3 and 5 bar of brake mean effective pressures (BMEPs). At a BMEP of 3 bar, the maximum hydrogen energy share (HES) could be reached to 50.1% without erratic engine operation, the level of NO, HC, CO and CO2 emissions were decreased up to 18%, 62%, 56% and 45% as compared with neat diesel case. However, there was no significant change in brake thermal efficiency (BTE) and smoke at high HES. At a BMEP of 5 bar, the maximum possible hydrogen energy share was nearly 40%, beyond this energy share severe knocking was noticed. NO emission was raised by 48%, whereas smoke and CO2 emission were decreased up to 20% and 24%. The brake specific HC emission was increased by 42% and there was no significant variation in CO emission with HES