This research was conducted to study the influence of the addition of kersen leaf extract, to understand the variation of temperature effect, and to determine the optimum result of hydrogen production in the process of water electrolysis within the kersen leaf extract using voltammetry method. The use of voltammetry cyclic method in water electrolysis was performed by setting the sweeping rate 50 mV/s. NaHCO 3 solution is functioned as electrolyte solution with a concentration of 5 g/L. A media modification which is a kersen leaf extract was added with concentration variety of 0; 0,2; 0,04; 0,008; 0,0016; 0,00032; and 0,000064 g/ml in temperature variety from 25°C, 35°C, and 45°C. The addition of media can increase hydrogen gas production. Hydrogen gas is produced higher at 45°C than it is at 25°C and 35°C. An optimum condition is obtained from sample 4 with 0,008 g/ml concentration.