Ultrashort pulsed laser has been expected to be a powerful and reliable tool for micro-welding of glass. Picosecond pulsed laser with high pulse repetition rate leads to melting of glass at the vicinity of focal region by the heat accumulation, in which the absorption point of laser energy moves periodically in beam axis. Thus, the control of focusing situation in beam axis is very important to investigate the mechanical strength of weld part. In the microwelding of borosilicate glass by picosecond pulsed laser of 1064 nm, characteristics of molten area creation and weld joint were discussed. Numerical aperture (N.A.) greatly affects characteristics of molten area formation, and superior focusing characteristics, such as N.A. 0.65 enable a long region of high power density in beam axis, which can create a large molten area without cracks even under high energy condition. An appropriately large molten area inside glass has high mechanical strength, when continuous and large molten areas were formed. In addition, high density and large size of molten area without crack led to increasing Young's modulus, and uniform and high Young's modulus of molten area results in higher breaking stress.