The aim of this study is to analysis the effect of MgO on strength development and microstructure of alkali-activated slag (AAS) in air curing condition. Four mixtures of AAS are prepared using different MgO content (0%, 5%, 10%, and 15 % by weight of slag) at water to binder ratio of 0.4. The flow, compressive strength, scanning electron microscopy, and X-ray diffraction are tested under relevant standards. The addition of MgO significantly accelerated the hydration rate of AAS. AAS with adding MgO tended to increase the compressive strength and to reduce the flow. The higher adding MgO content was associated with higher hydrotalcite-like phase (Ht) formation which improved the microstrure of AAS in the air curing condition