A simple method is proposed for determining the mode coupling coefficient D in step-index multimode optical fibers. It only requires observation of the far-field output pattern for one fiber length with the input light launched centrally along the fiber axis (theta(0)=0). For illustration, the coupling coefficient determined by this simple method for a step-index plastic optical fiber was used to calculate the coupling length L(c) at which the equilibrium mode distribution is achieved, and length z(s) at which the steady-state distribution is achieved. Our results are in good agreement with experimental results reported earlier.