Objective To investigate the mechanical character, microleakage and mineralizing potential of nanohydroxyapati（te nano- HAP）- added glass ionomer ceme（nt GIC）. Methods 8% nano- HAP were incorporated into GIC as composite, and pure GIC as control. Both types of material were used to make 20 cylinders respectively in order to detect three- point flexural strength and compressive strength. Class Ⅴ cavities were prepared in 120 molars extracted for orthodontic treatment, then were filled by two kinds of material. The microleakage at the composite- dentine interface was observed with confocal laser scanning microscop（e CLSM）after stained with 1% rhodamin- B- isothiocyanate for 24 hours. Class Ⅴ cavities were prepared in the molars of 4 healthy dogs, filled with composite, and the same molars in the other side were filled with GIC as control. The teeth were extracted to observe the mineralizing property with polarimetric microscope in 8 weeks after filling. Results Three- point flexural strength and compressive of nano- HAP- added GIC were increased compared with pure GIC（ P
