Magnetic behaviour of antiferromagnetic monolayer under exteal field is studied. This is the first time to calculate all components of spin statistical average of an antiferromagnetic system with the random-phase approximation. To do so, a method is developed by many-body Greens function theory. Magnetization and susceptibility are investigated when exteal field is applied in either the x- or z-direction. The results are compared with the ferromagnetic monolayer.