At high temperatures the Al-Mg-Si alloy structure is characterized by low resistance due to the complex oxidation effects involving the formation of layers or scale oxides and internal degradation. The situation becomes more complex when this alloy undergoes a welding process of metal bonding that involves heating and cooling. Subsequently, the welded structure is exposed to high temperature oxidation resulting in different oxide formation and phase changes. Therefore, the present study aims to determine the effect of high temperature oxidation on Al-Mg-Si alloy welded joint using ER 4043 (Al-5% Si) filler metal. The analysis and characterization of samples include morphology and microstructure using OM and SEM / EDX, and the oxide phase was determined using XRD. The addition of 5% Si from the filler metal and the formation of SiO2 on the surface of the welded structure due to the oxidation exposure was found to be effective in lowering the oxidation level by reducing penetration of oxygen diffusion into the alloy substrate. SiO2 also helps strengthen the Al2O3 protective layer. In conclusion, the use of an ER 4043 as a filler metal containing a 5% of Si as a major secondary element in the Al-Mg-Si welded structure is suitable for high temperature applications.