Glass ionomer cement (GIC) is one of the ideal dental cements because it is easy to use for filling, lining and adhesion restoration. GICs have good biocompatibility, caries prevention potential, chemical adhesion to mineralized tissues, but the disadvantage is poor mechanical properties. Hydroxyapatite (HA) has good biological properties, and could improve the mechanical properties of materials when added with the appropriate content. This paper will present the effect of adding HA powder at 5, 10, 15% mass into glass powder on the compressive strength of GIC materials when immersed in three environments: deionized water (DW), artificial saliva (AS), and simulated body fluid (SBF) and other properties. The results showed that when the HA powder was added, the working time and setting time of GIC mortar lasted longer, and the opacity of hardened GICs was increased. The change in compressive strength according to the curing time in three environments of each GIC sample was similar, but the compressive strength as of the sample soaked in SBF was higher than in DW and AS. The supplementary of HA at 10% and 15% content reduced the compressive strength compared to the control sample B. With the GIC sample HA05, the suitable amount of HA addition at the 5% content improved the compressive strength, especially the 28-day strength. The apparent density of the sample HA05 was higher than that of control sample B. SEM images and EDX results showed that the cured GIC sample HA05 had a more dense structure and the appearance of characterized cotton crystal cluster of HA with the Ca/P ratio = 1.68 on the surface of the soaked sample in SBF. These results demonstrated the role of HA in improving the compressive strength and the bioactivity of GIC materials.