Abstract: Femtosecond laser is a promising tool for micro/nano joining due to its low heat input and small heat affected zone. In this paper, fused silica was welded by using femtosecond laser. The shear strength and failure modes of the joints were investigated and the joint formation was studied. The shear strengths of the glass joint were between 6.4 MPa and 40.4 MPa, depending on the welding parameters. It was found that the gap of the faying surfaces between the two glasses had significantly effect on splashing, which would lower the joint strength. The results also indicated that high average power of laser and small distance between two weld seams tended to induce cracks in the glass, which were crack initiation sites during shear test and reduced the joint strength by half, compared with the joints with no cracks.