The purpose of this study was to estimate the changes of marginal adaptation to the cavity floor of light-cured glass ionomer cement base after application of a composite restoration. Eighty non-carious extracted human molars were used in the present study. Circular cavities were prepared on the center of the exposed dentin surface to 0.5mm, 1.0mm, 1.5mm, 2.0mm in depth and the prepared cavities were pretreated with Dentin conditioner and filled with Fuji II LC(GC Int. Co., Japan). They randomly assigned into 3 groups according to the difference in application of a composite restoration; Group 1(control group): only glass ionomer base, Group 2: The application of a composite restoration surrounded by dentin with class I cavity over glass ionomer base after conventional dentin bonding to the exposed dentin and glass ionomer base, Group 3: The application of composite restoration not-surrounded by dentin over glass ionomer base after conventional dentin bonding to the exposed dentin and glass ionomer base. To examine the interface between cavity floor and light-cured glass ionomer cement base, each groups were sectioned vertically through the center of restorations with diamond saw and the gap size(${mu}m$) of interface measured by SEM. The results were analyzed by using One Way ANOVA. The results were as follows: 1. Good adaptation between glass ionomer cement base and cavity floor was showed in specimens with 0.5mm, 1.0mm depth base of control group. But in specimens with 1.5mm, 2.0mm depth base of control group, the gap was measured about $15{mu}m$, $40{mu}m$ respectively. 2. Gap size in group 2 was significantly higher than that in control group(P<0.05). 3. Gap size in group 3 was significantly higher than that in control group and group 2(P<0.05). 4. It was possible to observe the good adaptation between glass ionomer cement base and dentin which was intermediated with 4-10${mu}m$ hybrid layer in specimens with 0.5mm, 1.0mm depth base of control group. Cohesive fracture within cement base was observed in all specimens which had the gap between glass ionomer cement base & dentin. 5. It was possible to observe the gap formation between cement base and bonding agent and between composite resin and dentin in all specimens of group 2.