Propolis is a natural material which has different biological activities such as antibacterial, antifungal and anti-inflammatory action. The aim of the current study was to investigate the effect of Propolis lining material on marginal leakage of resin modified glass ionomer. The degree of microleakage at different observation periods, 1 month, 3 months and 6 months were evaluated.Six young adult dogs were experimented with, using a sum of sixty sound teeth. Standardized class V cavity preparations were prepared on the facial surfaces of the selected teeth. All teeth were conditioned by 10% polyacrylic acid for 10s, then rinsed for 10s and dryed. All the cavities were restored with resin modified glass ionomer while in half of them Propolis was used as lining material under resin modified glass ionomer. After observation periods, dogs were sacrificed and then the teeth were separated and prepared to be tested. Statistically significant difference between samples restored with Propolis and without Propolis. However, no statistically significant difference was reported between occlusal and gingival sides of all groups. Also a statistically significant difference was found between samples stored at different time intervals.It was concluded that Propolis used as lining prevented leakage under resin modified glass ionomer and that Propolis appeared to have relative durability in vivo after the tested periods.Further clinical investigations to assess more beneficial uses of Propolis are recommended.