The effect of suction/injection on transient free-convective flow bounded by two infinite vertical parallel porous plates in the presence of thermal radiation is investigated numerically as well as analytically. The transient mathematical model has been solved using the implicit finite difference method while the steady state version of the physical situation has been solved using perturbation method. During the course of numerical computation, an excellent agreement was found between transient solution at large value of time and steady state solution. In addition, it was found that the time required to reach steady state is directly proportional to the Prandtl number of the working fluid for fixed values of other controlling parameters.