Two kinds of gravity gradiometer are described in this paper: one is laser interferometer and the other is atom interferometer. The authors have found that free fall gradiometery offers much promise for dynamic measurements and the free fall system can either use macroscopic objects in free-fall (mirrors) or atomic fountains. The two systems have similar intrinsic sensitivities. Based on the initial gravity gradient measuring experiment with NIM-Ⅱ, the authors put forward two ideas to design the gravity gradiometer: Single-Interferometer and Dual-Interferometer. The authors favor the second one in that it can satisfy the precision requirement at the same size as the traditional absolute gradiometer.