Effects of split injection, with a relatively short time interval between the two sprays, on the spray development process, and the air entrainment into the spray, were investigated by using laser induced fluorescence and particle image velocimetry (LIF-PIV) techniques. The velocities of the spray and the ambient air were measured. The cumulative mass of the ambient air entrained into the spray was calculated by using the entrainment velocity normal to the spray boundary. The vortex structure of the spray, formed around the leading edge of the spray, showed a true rotating flow motion at low ambient pressures of 0.1 MPa, whereas at 0.4 MPa, it was not a true rotating flow, but a phenomenon of the small droplets separating from the leading edge of the spray and falling behind, due to air resistance. The development processes of the 2nd spray were considerably different from that of the 1st spray because the 2nd spray was injected into the flow fields formed by the 1st spray. Therefore, the status of the flow field right before the 2nd spray injection, including the ambient air motion formed by the 1st spray, should be considered in the development process of the 2nd spray. Consequently, the development processes of the 2nd spray cannot be predicted without prior analysis of the effect of the 1st spray. In addition, the total mass of the ambient air entrained in the spray was, on average, 23.7% greater at low ambient pressures of 0.1 MPa than at higher ambient pressures of 0.4 MPa, for the three injection conditions tested.