Aluminum laser ablation by using a Nd:YAG laser with the wavelength of 1,064nm is numerically simulated to examine the mechanisms of the laser ablative impulse generation. Different conditions for ambient gas pressure ranging from 10-2 to 100kPa are analyzed using a computational fluid dynamic technique which incorporates the physical modeling for laser ablation phenomenon such as the heating of a solid target including melting and evaporating process, the expansion of the vaporized plume into the surroundings, and the laser absorption of the ambient gas and the vaporized aluminum. The calculated impulse data is compared with the time-resolved impulse data measured by using a velocity interferometer named VISAR (Velocity Interferometer System for Any Reflector). A fair agreement is obtained between the present method and the measurement. The present result shows that the laser absorption by the dissociated and ionized ambient gas is essential to explain the impulse generation by the laser irradiation of aluminum for the case of 100kPa.