The purposes of this study were to examine applicability of limestone as a conditioning agent and to determine the optimum conditions for improving dewaterability of digested sludge. The optimum conditions for temp., pH, dosage, particle size, time, and pressure was $30^{circ}C$, 7, 45 g/l, 100 mesh, 30 min., and $2.66 { imes} 10^4 N/m^2$, respectively. On the basis of induced optimum conditions, if adding limestone in digested sludge, it estimated that its specific resistance was $1.43 { imes} 1012 m/kg$. Therefore, it seemed that we could utilize limestone as conditioning agent for improving dewaterability of digested sludge.