Betalain is the pigment which is found in reed beet. Betalain in red beet can be used as antioxidant. The purpose of this experiment was to determine stability of antioxidant activity from red beet extract using solvent combination of ethanol and ethyl acetate by ratio 100 : 0, 80 : 20, 60 : 40, 40 : 60, 20 : 80, and 0 : 100, combination of extraction temperature are 26°C, 40°C, and 50°C and time are 3, 6, 9 hour. The obtained of red beet extract using the best combination of solvent and time extraction will used again in RSM (Response Surface Methodology). Stability of antioxidant activity from red beet extract was determine by IC50, fenolic in total, color spectrum. The result showed that ethanol 100 has the lowest IC50, highest fenolic in total, rendement and colour spectrum. That best extract of red beet is used in RSM to find the best stability of antioxidant activity and fenolic in total against pH and temperature. pH which used are 4, 5, 6, 7, 8 and temperature are 65°C, 75°C, 85°C and 95°. The result showed pH 4 and 65°C temperature is the best combination who had a lowest IC50 and highest fenolic in total