Femtosecond laser has distinct advantages in the field of micro/nano joining because of its unique properties, such as low heat input, nonlinear absorption and almost no heat affected zone. Silica glass and silicon are welded using 800 nm femtosecond laser with power of 4~30 mW and frequency of 1 kHz. The shear strength and failure modes of the joints are investigated, the joint cross sections processed by etching are observed, and the fracture surface is also studied. The shear strength of the joints is in a range of 7~54 MPa, depending on the welding parameters. It is found that the laser power, scanning speed, numerical aperture and defocusing length have significant effect on the joint strength. The results indicate that focusing the laser on the interface can obtain high shear strength joints with appropriate laser power and scanning speed.