This patent discloses a method and a system of radio resource allocation for a mobile MIMO-OFDMA system, the method comprising the steps of: generating a diagonal matrix according to a channel matrix; determining a sum capacity function associated with the diagonal matrix, a plurality of transmission powers, and a plurality of subcarrier fractions; performing a first subcarrier fraction allocation by maximizing the sum capacity function according to a predetermined user capacity condition; performing a first transmission power allocation according to the first subcarrier allocation; and performing a second subcarrier fraction allocation by maximizing the sum capacity function according to the first transmission power allocation and the predetermined user capacity condition.