Abstract: Using 3D admittance analysis technology，the state of gravity isostasv on Manihiki Plateau was studied with its gravity anomalies and bathvmetrv. The tectonics of the plateau was analyzedbased on the isostatic results and previous research including seismic studies. The relatively low coherence observed between gravity and bathvmetrv at long wavelength(>100km) may indicate the existence of a low density anomaly beneath the plateau crust. According to the results from airy isostatic analysis，this anomaly is not completely caused by as isostatic adjustment of the oceanic crust to theplateau topography，there must be mass loading beneath the plateau crust causing by an underplating of magma. At short wavelengths，the topography conforms to the airy model. The crust thickness ofthe plateau is 22. 7km，consistent with the seismic research result. A lithospheric flexural isostatic analvsis showed that the effective elastic thickness of lithosphere around the Manihiki plateau is 2. 5-5. 5 km，with an optimal value of 3. 5 km. This small elastic thickness may means the plateau is emplaced on a young lithosphere. A flexural model takes into account the mass loading beneath the plateau and the scale of the bottom load was estimated quantitatively. It demonstrates that the mean depth of the bottom load is 18km，the magnitude is about 30%of the topography，and the volume is about9. 3 X 10' km.Thus the bottom load is an important part of the plateau crust，while the underplating of bottom load plays an important role in the evolution of the plateau.