Abstract: In the view of low thermal efficiency and high emissions of spark ignition (SI) engines, this paper investigated the effect of hydrogen blending at intake ports to an SI engine on improving its economic and emissions performance. The experiment was carried out on a modified 4-cylinder SI engine on which hydrogen could be injected into the intake ports sequentially via an ECU (Electronic control unit) based control system and mixed with gasoline on-line. The combustion and emissions characteristics of a hydrogen-blended IC engine were investigated under the conditions of 1 500 r/min and the stoichiometric equivalence ratio with four different hydrogen volumetric fractions of 1%, 1.5 %, 2 %, 3 %. The test results demonstrated that, compared with the original engine, the brake engine thermal efficiency was increased by 3.23 % and the cycleby-cycle variations of the peak in-cylinder pressure was reduced by 3.18 % on average at the hydrogen addition fraction of 3%. Ignition delay and the rapid burning duration were shortened; the peak in-cylinder pressure was increased; CO₂ and HC emissions were obviously lower with the increasing fraction of hydrogen in the intake. However, NO_x and CO emissions were also increased. In a word, the addition of hydrogen to a gasoline-fuelled IC engine is a good way to improve engine performance.