Reducing CO2 in nature is the reason for the development of alternative technologies such as mortar geopolymers. This will have an impact on reducing the use of Portland cement. Moreover, the potential of local materials is also feasible to be developed especially contains an ingredient that can support environmentally friendly alternative materials. This study aims to determine the mechanical behavior of geopolymer mortars that consist of sand, fly ash type F from Tanjungjati Jepara, white soil from Kupang NTT (as substitution of fly ash), and Potassium Hydroxide (KOH) 8M as an activator. The specimens were cube 5x5x5 cm3 and molds similar to number eight. The mix proportion was 1 binder : 3 sand. Binder is a mixture of fly ash, white soil, and KOH 8M solution with w/b of 0.5. Mortar dried in the oven until 24 hours at 60 °C. Specimens were tested for compressive and tensile strength, density, and porosity within a specified time. The results show that the substitution of white soil in geopolymer mortar can increase of the compressive and tensile strength, increase porosity, and make the weight of volume the mortar become lighter. The substitution of white soil at 10, 20, and 25% shows a trend that has a better result than the others, while the substitution of 5% has not been effected yet.