Polycrystalline bulk sample Zn_0.99Fe_0.01O was fabricated by a solid-state reaction method and modified by high-pressure treatment technique at a pressure of 5GPa. The structure, morphology and magnetic properties of these samples were investigated in order to clarify the effect of pressure on magnetism of Zn-Fe-O system. It is found that the particle size of the modified samples becomes larger as well as the physical contact between neighboring particles becomes better. All samples show obvious ferromagnetic behaviors at room temperature, and the magnetization of modified samples greatly increases. It is believed that the larger particle size and the closer contact between neighbouring particles resulted from high-pressure treatment cause stronger ferromagnetic exchange interaction in Zn-Fe-O system.