In this paper, full bottom dielectric isolation (BDI) is first demonstrated on horizontally stacked Nanosheet device structures with Lmetal 12 nm. The comparison of full BDI scheme vs punch through stopper (PTS) scheme has been systematically studied. By comparing off-state leakage current, short channel behavior and effective capacitance (Ceff) for both schemes, we show that BDI could potentially provide: 1) good immunity of sub-channel leakage due to process variation (from parasitic "fat-Fin" which is unique in Nanosheet structure); 2) power-performance co-optimization.