Adding temper into clay prevents to shrink and crack, or twist the clay during the dry process. The purpose of this study is to identify the role of temper and the physical property of earthenware coffin according to the clay state modification in the reduction firing based on earthenware coffins found in Naju which had been made in Three Kingdom Period. Clay from the Oryang-dong site in Naju was used as circle shaped samples. The samples were tempered with various proportions (0%, 20%, 40%) and fired in various temperature ($1000^{circ}C$, $1100^{circ}C$, $1200^{circ}C$). Physical properties were identified by macroscopy, an optical microscope (x50), specific gravity and porosity. Chemical components and existence of specific minerals were identified by analysing XRF and XRD. As a result, the more firing temperature increase, clay volume expansion. To prevent the volume expansion, temper was added.