The purpose of this study is to evaluate the effect of filler metals in the gas metal arc welding (GMAW) process on the Aluminium alloy of AA 6061-T6 welded joints. 6 mm thickness plate with single V butt configuration was used and welded using two different fillers ER 4043 and ER 5356 and controlled welding parameters. The relationship between hardness and microstructure of the welded parts were studied and compared and the results showed that the post weld mechanical property has decreased especially for the filler ER 5356. The hardness at the heat affected zone (HAZ) of ER 5356 also has reduced almost 85% compared to the hardness of AA 6061 base metal. The microstructure of ER 4043 welding material shows the shape of columnar grains and dendrite structure. Microstructure morphology of ER 5356 welding material shows dendrites and intermetallic particles (Mg2Si) have partially dissolved and scattered in the welding material.