The radiopacity of glass ionomer cements is quite variable. The use of a poorly radiopaque material as a base under other restorative materials can mislead the dentist to a diagnosis of recurrent decay. This study investigates the radiopacity of these materials and proposes a minimal radiopacity under which a material should not be used as a base or liner. It is important to determine the radiopacity of glass ionomer dental materials so that the clinician can appreciate the type of restorative materials used when radiographically evaluation the possibility of recurrent dental caries. In this study, radiopacity of Vitrement and Chemfil was compared with that of Cavalite, Miracle mix and polycarboxylate cement. Tooth model of artificial cavity preparation for diagnosis of recurrent caries was omitted. Radiopacity of each material was measured using relatives between thickness and radiopacity of Aluminium step wedge. The results were as follows : 1. Radiopacity of Vitrement was some higher than enamel. 2. Chemfil, restorative glass ionomer, was less radiopaque than enamel. 3. In order of higher radiopacity than enamel, Miracle mix was highest and was followed by polycarboxylate cement, Cavalite and Vitremer. 4. Vitremer, the Glass Ionomer Cement, is useful to detection of recurrent caries, because it is slightly higher radiopaque than enamel. So, it is suitable for restorative material and luting cement.