The use of advanced high strength steel (AHSS) sheets has been acknowledged as an important solution for vehicle weight reduction, and thus, carbon dioxide emission reduction. The development of third-generation AHSS has become one of the steel industry's most prominent concerns in recent years. However, the selective oxidation of alloy components such as silicon and manganese makes obtaining high-quality hot-dipped galvanized steel sheets extremely difficult. To determine an optimal process window for controlling the surface microstructure of AHSS, the effect of dew point on selective oxidation of silicon and manganese, and decarburization in a 0.2%C-1.5%Si-2.5%Mn (mass fraction) steel sheet was studied by performing continuous annealing simulation experiments. Glow discharge optical emission spectrometry (GD-OES) was used to determine the depth profiles of alloy elements, and SEM and OM were used to determine the depths of internal oxidation and decarburization zones in the subsurface. The surface and internal oxides' precise microstructures were studied using TEM on a FIB-prepared cross-sectional specimen. The increasing dew point of the atmosphere through the heating and soaking section portion of continuous annealing results in the transformation of external oxidation of silicon and manganese to internal oxidation. When the steel was annealed in an environment with a dew point of -40^oC, a continuous silicon, manganese external oxidation layer with an average thickness of 40-50 nm covered the surface. When the dew point was elevated to +10^oC, a subsurface oxidation layer approximately 5-μm thick formed. Due to the substantially lower oxygen pressure required for the Si/SiO₂ equilibrium, the internal oxides exhibited a core-shell structure consisting of a Si-rich oxide core and a surrounding Mn-Si mixed oxide shell. A higher dew point results in the formation of an obvious decarburization layer in the subsurface, which is visible as a layer of ferrite grains with significantly decreased microhardness. When the dew point was increased from -40^oC to +10^oC, the thickness of the decarburized zone increased from 0 μm to 45 μm, and the C content of the decarburized zone decreased from 0.18% to 0.01%. External oxidation can no longer be decreased further by increasing the dew point, yet the depth of internal oxidation and decarburization in the subsurface continues to increase. Therefore, maintaining an appropriate dew point range for the annealing atmosphere is necessary to manage external oxidation and decarburization. The optimal dew point should be adjusted between -20^oC and -10^oC when annealed at 870^oC for 120 s in 5%H₂-N₂ atmosphere.