A free piston linear engine (FPLE) with glow plug ignition was designed and manufactured. This paper investigated effects of hydrogen addition on the FPLE performance and emission characteristics. Experimental results showed that the FPLE can be started manually and then steady FPLE operating state is achieved. Compared with the crankshaft model engine, the FPLE has a greater speed around the top dead centre. The peak acceleration of the FPLE (4100 m/s(2)) is more than three times greater than that of the crankshaft model engine. Results also showed that the FPLE performance including the indicated power, indicated thermal efficiency and indicated mean effective pressure improves remarkably with hydrogen addition. The optimal FPLE performance is achieved when the hydrogen volume fraction is 4%. However, the engine performance cannot be further improved when the hydrogen volume fraction continues to increase due to the occurrence of overadvanced combustion phasing. Moreover, the FPLE operating range is significantly broaden and the operating stability is improved with hydrogen addition since the fast burning speed and wide flammability of hydrogen. Besides, the heat release rate, incylinder peak pressure and pressure rise rate are also obviously enhanced. The highspeed flame images showed the flame color becomes lighter and the yellow area is smaller compared with that of pure methane condition, indicating that the hydrogen addition to the fuel contributes to burn completely and reduce the soot formation. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.