The ordinary Portland cement (OPC) is most widely utilized construction materials which is main raw material for concrete and block production. However, its production consumes too much energy and release greenhouse gas (CO2). Geopolymers are the best cement substitute construction material alternative. Aluminosilicate and alkali solution are raw materials of Geopolymers. Fly ash is the most widely known aluminosilicate compounds, which is industrial waste that pollute the environment cause health problems in human respiratory organ, pollute water bodies and destroy aquatic life. However, which can be used as the raw materials in geopolymer production. This study investigates the effect of controlling variables on the compressive, tensile strengths, water absorption and mechanical strength per density of sisal fiber Reinforced geopolymer Composites materials (SFRGCM). A Response Surface Methodology (RSM) with box Behnken design (BBD) was employed to characterize and obtain the significance level of the selected variables and the points at which optimum values achieved. Screened controlling variables and their upper and lower limits were, sisal fiber volume percentage (5% to 15%), sisal fiber length (5mm to 35mm) and alkali concentration (8M to 16 M). The results proved that best compressive strength, splitting tensile strength and water absorption obtained was 21.5 Mpa, 4.6 Mpa and 5.325%. The point at which the result achieved were sisal fiber volume percentage, sisal fiber length and alkali concentration of (5%, 20mm and 8M), (15%, 35mm and 12M) and (5%, 20mm and 8M) respectively. the best results of compressive strength per density and splitting tensile strength per density of SFRGCC was 23.8 Mpa/(g/cm 3 ) and 4.9 Mpa/(g/cm 3 ) respectively. The density at the point was 0.9035 g/cm 3 and the density at the point was 0.941 g/cm 3 respectively. Key words: Fly Ash, Geopolymer, Alkali Activators, Sisal Fiber, Alkali Concentration, Mechanical properties