The objectives of this in vitro study was to assess the microleakage of encapsulated versus hand-mixed glass ionomer restorations. Eighty-four extracted human primary molars were randomly allocated into 6 groups. Each sample was prepared for Class II slot cavity and then restored with six different materials: Fuji IX GP capsule^®, Ketac™ Molar Aplicap™, RIVA SC capsule^®, Fuji IX GP^®, Ketac™ Molar, RIVA SC^®. The procedure of each glass ionomer restoration followed manufacturer’s instructions. All teeth were thermocycling for 500 cycles between 5°C and 55°C. After thermocycling the whole surface of each tooth was coated with nail varnish except for one millimeter around the restoration. The teeth were immersed in 0.5 % methylene blue for 4 hours and then sectioned mesiodistally. The sections were analyzed for microleakage under X30 magnification of a stereomicroscope. The median of the percentage of dye penetration between encapsulated and hand-mixed glass ionomer restorations were analyzed by the Mann-Whitney U statistic at 0.05 a level. The result showed the microleakage of encapsulated groups different from those of hand-mixed groups. However, Ketac™ Molar Aplicap™ encapsulated group showed statistically significant less than Ketac™ Molar (p = 0.002). But there were no statistically significant of microleakage between Fuji IX GP capsule^® and Fuji IX GP^® (p = 0.773), and there were no statistically significant of microleakage between RIVA SC capsule^® and RIVA SC^® (p = 0.207). In conclusion, the microleakage of encapsulated Ketac™ Molar Aplicap™ group appeared to be less than handy-mixed Ketac™ Molar group