Nuclear fusion rates in a muonic tritium molecule are calculated by the coupled rearrangement channel method. The interaction between two tritons is described by the optical model. The optical potential is determined by reproducing the t + t → α + n + n reaction cross section. The nuclear fusion rate obtained was in good agreement with an experimental value. The charge symmetry of nucleons is partially broken in the low energy t + t reaction by 30%. The mechanism of the reaction is approximately described by the proton stripping reaction. We deny the possibility of the fusion through a negative parity resonant state.