Foamed concrete is one of the lightweight concrete innovations without coarse aggregates by adding foam to concrete mortar mixtures. The concrete was developed as a non-structural element in the building. This research utilizes rice husk ash (RHA) as a substitute material for cement. This study determines the effect of foam percentage and the use of rice husk ash (RHA) on foamed concrete's microstructures. The percentage of used foam is 30%, 40%, 50%, and 60%, while the percentage of RHA substitution is 5%, 10%, 15% and 20% on cement. The use of RHA is to show the secondary reaction of cement to make new CSH. Tests conducted in this study include fresh concrete test, the compressive strength of 28 days, and SEM test. The fresh concrete test includes the slump flow test and setting time. Research shows that the value of slump flow decreases with increasing percentage of foam and RHA substitution on cement. Percentage foam 30% to concrete volume with RHA 0% shows the maximum slump flow value that is 63.25 cm. Setting time test results are increasing as the percentage of foam and substitution of RHA increases. The maximum setting time value is on 60% foam to concrete volume with RHA 20%. Percentage foam 30% with RHA 10% showed the maximum compressive strength value of 28 days at 7.77 MPa with density 1,378 kg/m 3 and air void/bubble structure with the smallest diameter.