The 257 nm photodissociation dynamics of CH3I adsorbed on a MgO(001) surface is studied using classical molecular dynamics method. The substrate is modeled by a 6X6X3 slab of movable ions surrounded by a semi-infinite array of static ions. A single adsorbate molecule is aligned with the surface normal, the methyl end pointed either toward or away from the substrate. The system is equilibrated by using a Monte Carlo method to obtain the starting configuration. Fragment final state distributions are calculated for kinetic energy, angle of departure, and rovibrational states. Upon photodissociation of the adsorbate with the methyl end pointed toward the surface, the methyl fragments experienced vibrational cooling, in agreement with experimental results. Some rotational excitation is predicted for fragments produced from the methyl down orientation. The kinetic energy distributions of both the methyl and iodine fragments are qualitatively similar to those obtained by experiment. The results are compared with those obtained by the same model for CH3I adsorbed on LiF(001). Trapping of iodine atoms by the surface has also been investigated in this simulation.