Unsteady vaporization of a metal target due to nanosecond pulsed laser heating when an ambient gas of finite pressure exists was studied numerically. A one dimensional model for target heating and for the gasdynamic flow of the vapor was employed including an analysis of the vapor motion inside the Knudsen Layer (KL). For single pulse laser heating of an aluminum target, it was found that the threshold laser fluence for evaporation of the target is greatly dependent upon the target surface reflectivity. The threshold laser fluence predicted by a thermal evaporation model is larger than the values obtained experimentally. The local Mach number of the vapor at the edge of the KL becomes equal to unity at the beginning of evaporation only when the ambient pressure is sufficiently small; for large ambient pressures, the Mach number of the vapor during the early period of evaporation decreases first and then increases to a local sound velocity. An acceleration of the gasdynamic flow during the early period of evaporation was found and attributed to the unsteady adiabatic expansion.