Purpose: Assessing the microleakage of glass ionomer restorations (GIC) prepared with both conventional and Er:YAG laser techniques in Class V cavities is the purpose of the study. Material and Methods: Twenty teeth with Class V cavities (4mm wide, 3mm high, 3mm deep) were used. Cavities were prepared on the buccal and lingual surfaces of teeth which were randomly separated into four groups. Er:YAG laser was used to prepare cavities in Groups II and IV, conventionally diamond bur was used to prepare cavities in Groups I and III. Then, Groups I and II were restored with conventional GIC (Ketac Molar Easymix, 3MEspe, Seefeld, Germany) and high viscosity GIC with resin coating (EQUIA Fil, GC Dental Co., Tokyo, Japan) to restore Groups III and IV. Samples were stored in distilled water at 37°C for 24 hours before subjecting to thermo-cycling (500 cycles,5°C-55°C). In a chewing simulator, all teeth were exposed to 50.000 cycles. We used 0.5% aqueous basic fuchsin dye to stain the samples and sectioned them bucco-lingually. Scoring dye penetration was made under a stereomicroscope. The data were analyzed by Kruskal-Wallis One-Way ANOVA and Mann-Whitney U tests. Results: While the highest microleakage scores were obtained in the group prepared with Er: YAG laser and restored with conventional GIC the lowest microleakage scores were obtained in the group prepared with Er:YAG laser and restored with high viscosity GIC. When cavities prepared with Er:YAG laser or conventional method were restored with high viscosity GIC, no statistically significant difference was found between them (p> 0.05). Conclusion: High viscosity GIC can be preferred as a restorative material in the restorations of Class V cavities prepared using both conventional and Er:YAG lasers.