<p>Entrainment process of surrounding fluid for a diesel spray is investigated based on the momentum theory and 1-D simple spray model developed by Musculus et. al. Entrained fluid distribution along the spray axis is theoretically estimated. The results show that the absolute value of entrainment velocity depends on axis location and is inversely proportional to the distance from the nozzle. Therefore, the large amount of entrained fluid comes from the upstream region near field of nozzle. 1-D simple spray model is performed for the same target and the effect of the injection rate on entraining process is clarified.</p>