A simple scheme is presented for generating n-atom GHZ state with resonant interaction between two-lev- el atoms and a single-mode cavity field. In the proposed protocol, the cavity initially prepared in the vacuum state, it is injected three two-level atoms initially prepared in the excited state, the last two-level atom prepared in the ground state are injected into cavity with 3-photon resonant interaction. The detection of the cavity can collapse the atoms to the desired 4-atom GHZ state. Th...