The penetration of fuel spray under diesel engine conditions was calculated by solving the cross-sectionally averaged equations for unsteady two-phase flow. The effects of a variety of processes in the spray, e.g., air entrainment and droplet evaporation, on spray penetration are analyzed through the corresponding nondimensional parameters. The integrated two-phase flow equation system can further be simplified with the approximation of equal gas and liquid velocities, which are reached after a short atomization region for a complete spray. A closed-form solution for the penetration length was derived from the system of integrated equations using the quasi-steady-state assumption. This solution is compared with the well-known Wakuri correlation and the Hiroyasu correlation. The expression of the spray tip and droplet penetration lengths are also deduced for nonconstant injection velocity. The effects of ambient air density and droplet evaporation rate on spray penetration were investigated numerically, and the numerical results agree very well with many experiments from the literature.