The effects of the diesel pilot injection parameters on combustion of natural gas (NG) under diesel engine conditions were studied numerically using a three-dimensional (3-D) model and experimentally in a constant-volume combustion bomb (CVCB). The 3-D model, incorporating complex interaction between diesel and NG during combustion was used to predict the main combustion characteristics (pressure, temperature and species concentrations). Numerically, combustion pressure traces are validated satisfactorily by the corresponding experimental values. The 3-D simulations are then performed to investigate the effects of diesel pilot injection parameters on NG combustion under diesel conditions. The investigated parameters are nozzle hole size and injection pressure. Predicted results suggest that NG combustion under diesel conditions can be improved by applying an injector with a great number of small holes and also by increasing the injection pressure of pilot diesel spray.