Disclosed is a mobile internet system, and more particularly, to a method and apparatus for supporting a multicast broadcast service (MBS) over WIMAX. The MBS supporting method of the present invention comprises assigning a first GRE key to a first MBS service flow for a first MS, and delivering MBS contents having the first MBS service flow through the use of first GRE key; and if a second MBS service flow for a second MS is the same as the first MBS service flow, delivering MBS contents having the second MBS service flow through the use of first GRE key pre-assigned to the first MBS service flow.