The Goryeo roof tiles from the Oegol site in Tangjeong, Asan are classified into three groups in color such as gray, red yellow and gray-yellow groups, respectively. While each group of tiles shows characteristic specific gravity, absorption ratio, LOI and vitrification degree, mineral content and distribution, and chemical composition are generally homogeneous among all groups of roof tiles. Also, all roof tiles and soils from the site show similar geochemical behavior of elements and clay-mineralization degree. This indicates that the soil from the site is probable to be a raw material of the roof tiles. Firing temperature of the roof tiles is estimated as 950 to 1,050℃ for the gray group, 800 to 900℃ for the red yellow group, and 900 to 950℃ for the gray-yellow group. In conclusion, roof tiles from the Oegol site is interpreted to be made of local clay without additive minerals, applying various firing conditions and standardized purifying process of raw clay materials.