As is already reported new combustion method named PCC(Plume Ignition Combustion Concept), which realized combustion of plume of rich hydrogen mixture by igniting injected hydrogen jet in the period of or just after the injection, is effective to reduce NOx formation drastically without offering any trade-offs on thermal efficiency in high output power operation in direct injection hydrogen engines. In this study it was made clear that increased unburned hydrogen decreased thermal efficiency in high lambda premixed operating condition which is commonly used in part load operation of direct injection hydrogen engines. This is thought to be caused by partially incomplete flame propagation occurred in high lambda portion of the non-uniform mixture that presents with slowing burning even in hydrogen engines. It was also found that the PCC combustion was effective to improve thermal efficiency by suppressing increase in unburned hydrogen under the operating condition mentioned above. The effect is understood to be caused by reducing dispersion of hydrogen mixture to form high lambda that presents with the incomplete flame propagation.