To evaluate and compare the effect of two different drying agents (PrimaDry and Ethyl alcohol 95% concentration.) on microleakage of filled (Ultraseal XT plus) and unfilled (Helioseal) fissure sealants in vivo and in vitro. A total of 60 extracted human upper and lower first premolars from15 adult healthy patients were divided into 2 groups 30 in vivo (Teeth were extracted 3 months after sealant application), and 30 in vitro (Teeth were stored 3 months in normal saline) each group were subdivided into 2 subgroups 15 sealed with filled (Ultraseal XT plus), while the other 15 were sealed with unfilled (Helioseal), each subgroup was subdivided into 3 classes composed of (5) premolars according to drying agent used: B0) without drying agent. B1) PrimaDry B2) Ethyl alcohol 95% concentration. After extraction teeth were thoroughly washed with water, cleaned with non-fluoridated (FL) prophylaxis paste. The unsealed were sealed after 3 months of storage in saline solution (in vitro).The occlusal surfaces were opened using tapered diamond fissure burs confined to enamel. Fissure sealant application was done following the manufacturer's instructions. The drying agent was applied after etching and before sealant application. Dye penetration method with methylene blue dye was used for microleakage assessment. The specimens were immersed in 1% methylene blue for 24h in order to allow dye penetration into possible gaps in the tooth–sealant interface. The teeth were sectioned longitudinally in using diamond saw, each section cleaned, examined and photographed under stereomicroscope at ×20 magnifications to assess dye penetration at the margins of the restorations. Two-way ANOVA was used for statistical analysis. In vivo results revealed that unfilled (Helioseal) achieved significant less microleakage than filled (Ultraseal XT plus) without any drying agent and PrimaDry with either filled or unfilled achieved the least microleakage. While in vitro results showed that both PrimaDry with Filled (Ultraseal XT plus) and Ethyl alcohol 95% concentration with Unfilled (Helioseal) achieved the least microleakage without significant statistical difference. Use of Ultraseal XT plus system according to manufacturer's directions showed the least microleakage both in vivo and in vitro. Use of PrimaDry increased the efficiency of both filled and unfilled fissure sealant.