A lot of difference in mechanical and metallurgical properties will take place by joining the metal with fusion and solid state welding processes. In solid state welding, the base metal will not melt but it will recast during the welding (without filler metal). In case of fusion welding, the base metal will fuse and solidify during the welding process with the help of a filler metal. So far, in this research work, the AZ91 grade magnesium alloy of 6 mm thickness plates were welded by friction stir welding (Solid state) process and gas tungsten arc welding (Fusion) process. The various mechanical and metallurgical properties have been analyzed with advanced equipments and the integrity of the joints were analyzed with the help of optical microscope.