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Focusing condition such as numerical aperture (N.A.) has a great influence on the creation of molten area and the stable welding process in fusion micro-welding of glass. In this study, a picosecond pulsed laser of 1064 nm in wavelength and 12.5 ps in pulse duration was tightly focused inside a borosilicate glass using objective lenses of numerical apertures 0.45, 0.65, and 0.85 with spherical aberration correction. Influence of numerical aperture on molten area formation was experimentally investigated through analysis of focusing situation in glass, and movement of absorption point, and then molten area characteristics were discussed. It is concluded that N.A. of 0.65 with superior focusing characteristics can form a large and continuous molten area without cracks, which enables achievement of stable joining of glass material by picosecond pulsed laser.