The vibration characteristics of the grid type improved ground by Deep Mixing Method was studied using the acceleration records observed, and the seismic response of the improved grovnd computed by Finite Element Method using the equivalent linear method are compared with the records observed. The results are summarized as follows: (1) The grid type improved ground which consists of improved part having shape of grid and unimproved part surrounded by improved soil behaves like a rigid body altogether during an earthquake. (2) The maximum acceleration and dynamic displacement of the improved ground are almost equal to those of the sand layer under the improved ground. (3) The maximum acceleration of the improved ground are smaller than that of original ground around the improved ground. (4) There is a obvious phase difference between the acceleration of the improved ground and that of the original ground around the improved ground. (5) The computer program considering the energy dissipation into the underlaying midium was proved to be useful for calculating seismic response of improved ground.