Using hydrogen in SI engine as supplementary fuel is an effective way to improve engine performance. Taking hydrogen as supplementary fuel, the dual fuel injection system which was hydrogen/gasoline combined injection with gasoline port injection and hydrogen direct injection was proposed. Cold start characteristics of this dual fuel engine was studied experimentally. The study was performed with three injection modes which included pure gasoline port injection, pure hydrogen direct injection and hydrogen/gasoline combined injection. The test results showed that hydrogen addition improved ignition reliability and advanced the occurrence of the first effective firing cycle. The combined injection mode had larger adaptive range of ignition timing during cold start period. The optimum ignition timing for the maximum in-cylinder pressure was advanced because of the higher flame propagation speed of hydrogen. Comparing the three injection modes, the combustion and emissions performance of hydrogen/gasoline combined injection were superior to pure gasoline port injection; the CO emission and the HC emission were reduced by 40% and 58.3% in the case of combined fuel injection, respectively. However, because of the enhanced in-cylinder temperature, NOx emissions were increased after hydrogen addition. (C) 2016 Elsevier Ltd. All rights reserved.