Combustion characteristics and heat release analysis of a spark-ignited engine fueled with natural gas-hydrogen blends were investigated. For the same excess air ratio and at the optimum ignition timing setting, the natural gas combustion gives higher values of peak cylinder pressure and peak heat release rate in the case of the rich mixture combustion. In the case of the stoichiometric mixture combustion and the lean mixture combustion, the natural gas - hydrogen blend with a 10% hydrogen fraction gives the highest value of the peak cylinder pressure and the peak heat release rate. The initial combustion duration and the total combustion duration decrease with the increase of hydrogen fraction in the natural gas-hydrogen blend. The addition of hydrogen into natural gas decreases the ignition delay. Although the optimum ignition timing is retarded with the increase of the hydrogen fraction in the natural gas-hydrogen blends, the heat release process is not postponed.