304 stainless steel plates of 60 mm thick are welded using IPG YLS-6000 fiber laser and 5 mm wide groove. The microstructure and mechanical property of joint are tested by optical microscope, scanning electron microscope, energy disperse spectroscopy and tensile testing machine at low temperature. The result shows that complete defect free butt joints are obtained with ultra-narrow gap laser welding. The width of weld is uniform and is about 3.8 mm. Microstructure of joint are consist of columnar austenitic, the skeletal and the lath ferrite. There is a little ferrite diffuses and grows to the direction of the base metal and no obvious coarse grain in heat affected zone (HAZ). Main alloy elements such as Cr, Ni are even distributed in the weld and HAZ. The microhardness of the HAZ is slightly higher than that of weld metal and base metal. The tensile strength at temperature of 183 K, 213 K, 243 K and 273 K temperature are 86%, 96.7%, 94.5% and 92.6% of base metal, respectively, which increases when temperature decreases and the maximum is up to 1260 MPa. Samples are bended by 180° without any cracks or other defects.