The effect of defects (in the form of vacancies) on the transformation pressure of the Fm3m-Pm3m phase transition of KCl has been examined using molecular dynamics simulations. The simulated transformation has been found to be reversible, exhibiting a large hysteresis. Vacancy defects were systematically introduced into the crystal structure of the parent phase, and then a series of simulations using the Parrinello-Rahman method were performed to elucidate the transformation pressure. With an increase in the density of the defects, the hysteresis decreases. These results are entirely consistent with the observed behavior.