Abstract: Solid immersion lens (SIL) was employed to increase the optical writing density. The position of foci and numerical aperture of the focusing system with SIL were simulated, and the threedimensional optical data storage was conducted. The simulation result shows that the depth actually focused into the media is increased and the effective numerical aperture is enlarged with the suitable off-focus distance between the foci of focusing objectives and the bottom plane of SIL. With this result, a SIL with n=1.55 was positioned under a 0.55 NA micro objective to focus a 150 fs Ti∶sapphire pulse laser at 800nm, and a 5-layer reading and writing of data were achieved with 2.5mm separation between two layers and 0.6 mm separation between two bits. Accordingly, the storage density of 1.1×1012 bit/cm3 was obtained in this way.