The dynamical process of a two-step electron transfer (ET) occurring in the supramolecular triad system D-A(1)-A(2) embedded in a polar solvent is theoretically studied. The two-step ET is composed of the initial ET (D*-A(1)-A(2) --> D+-A(1)(-)-A(2)) and following two alternative ETs (forward ET, D+-A(1)(-)-A(2) --> D+-A(1)-A(2)- or backward ET,D+-A(1)-A(2) --> D-A(1)-A(2)). After the solvent free energy functions for the neutral and charged triad systems are expressed in terms of two reaction coordinates which represent the solvent polarization, the branching ratio between the alternative ETs is obtained from the dynamics of the Brownian particle on the free energy surface of the charged state D+-A(1)(-)-A(2) The branching ratio is calculated as a function of the free energy changes of the three ETs, and the values of the free energy changes that maximizes the yield of the forward ET are determined.