The study concentrates on the effects of structural parameters of an electronic fuel injector on its dynamic response (the opening delay and the closing delay). The injector was developed for a marine medium-speed diesel engine. The dynamic response from the start of the control signal to the end of the needle valve closing were investigated. Firstly, a complete and detailed model of the electronic fuel injector was built and integrated into an optimisation model, where a MOGA was applied. Secondly, the importance and effects of main structural parameters on dynamic response were examined, as were their interactions. Finally, a Pareto optimum was obtained through scattering charts and comparisons were made between the baseline design and the optimal design. Results show that the control piston diameter, fuel oil inlet passage diameter, fuel oil outlet passage diameter and their interactions are influential factors to the opening delay, while the fuel oil inlet passage diameter has the dominant effect on the closing delay. A small control piston diameter together with a small fuel oil inlet passage diameter contribute to a short opening delay, however, they lead to a significant increase in the closing delay. Moreover, a small closing delay prefers a large fuel oil inlet passage diameter. The selected Pareto optimum achieved a significant reduction in both the opening delay and the closing delay under three different rail pressures. (C) 2017 Elsevier Ltd. All rights reserved.