The aim of this study is to evaluate the effect of MgO on alkali-activated slag- rice husk ash paste. The mixtures were prepared with GGBFS replaced with10% RHA and modified by 2.5%, 5% and 7.5% MgO. Then these mixtures were compared to the reference mixture (without RHA and MgO content). The properties of paste were tested by flow, compressive strength, thermal conductivity and UPV analysis. In terms of finding, using RHA and MgO remarkably reduced the workability of AASR paste. In additions, the mere use of 10% RHA slightly reduced the strength of paste. However, adding MgO significantly accelerated the hydration of AASR samples in the early age and improved the strength and engineering properties of AASR paste samples.