This paper studied suitable methods to use reformed gas (H2 67% and CO 33% mixed), hydrogen as alternative fuels for an internal combustion engine. Both the internal mixture formation and the external mixture formation were studied. PIMGI (Port and Intake Manifold Gas Injection) engine of the external mixture formation was equipped with two electro-magnetic injectors per cylinder. One injector was installed in the intake manifold for the homogeneous mixture and the other was installed near intake port for stratified charge. The fuel pressure for PIMGI was 0.8 MPa. In GDIC (Gas Direct Injection into Cylinder) engine of the internal mixture formation, the gas injector driven by the hydraulic pressure was installed in the cylinder head. The fuel pressure for GDIC was 5.0 MPa. In the GDIC, the fuel was injected early after intake valve was closed because the gas pressure was relatively low for direct injection. Brake mean effective pressure of GDIC was 20 % higher than that of PIMGI, because the volumetric efficiency of GDIC was higher than that of PIMGI. However, the brake thermal efficiency of GDIC was lower than that of PIMGI entirely in case of using the same fuel The break thermal efficiency of the H2 engine was about 2% higher than the reformed gas engine. NOX emissions of MRG and H2 engines were approximately under 10 ppm at a lean burn process with excess air ratio λ >2.2. Both reformed and H2 gas engines realized high thermal efficiency and low NOX emissions under a fuel-lean condition.