In this paper, we propose a numerical formulation of decoupling matrix with rate one quasi-orthogonal space-time block code (QOSTBC) for eight transmit antennas. Tirkkonen et al. provided zero-forcing decoupling matrix for eliminating inter-symbol interference (ISI) in QOSTBC with 4 transmit antennas. We expand zero-forcing decoupling matrix with 8 transmit antennas, and also MMSE decoupling matrix with 4, 8 transmit antennas. Simulation results were provided to demonstrate that some SNR gains can be achieved by using decoupling matrix.