The author attempted to study the role of liners as a barrier against the penetration of the acids released from the liquid of dental cements. Six different kinds of liners were used in this study and the depth of penetration of the acids of phosphate and silicate cements were measured ands following conclusions were obtained. 1. All the liners successfully prevented penetration of the acids released from phosphate and silicate cements. 2. When silicate cements were placed into the cavity without any liners, free acids were liberated continuously and thus may give injuries to the pulp tissue.