Simulation of multi-band quantum transport based on a non-equilibrium Green's functions is presented in resonant tunneling diodes (RTD's), where realistic band structures and space charge effect are taken into account. To include realistic band structure, we have used a multi-band (MB) tight binding method with an sp3s* hybridization. As a result, we have found that the multiband nature significantly changes the results of conventional RTD simulations specifically for the case with indirect-gap barriers.