The increasing world’s population will make the energy needs consumption in the world increase, including Indonesia. The availability of petroleum in Indonesia so that to fulfill the petroleum need in this country, Indonesia should import oil from other countries. Because Indonesian’s depleation of energy reserves, it is needed for new and renewable energy that is able to meet domestic energy needs. One of new energy is bioethanol. Bioethanol is addressing of alcohol or etanol derived from biological materials; one of which material it is nira siwalan . Sap was originally only used as drink and sugar. If it is used as bioethanol production will increase the sale value. The purpose of this research was to determine the amount of yeast and optimal fermentation time in generating the highest volume and etanol content. The method used is fermentation using yeast saccharomyces cerevsiae as yeast a producer of ethanol and it was measured the decrease sugar, acidity, etanol volume, and etanol content as parameter study. The variables used were variations of yeast amount (0,5;1,0;1,5;2,0 grams) and variation of fermentation time (24,48,72, and 96 hours). Sugar reduction measure ment was done by use brix refractometer, degrees of acidity was measured using a pH meter, distillation volume of etanol was measured using the measuring cup, and ethanol was measured using a alcohol refractometer. The results is that the optimal amount of yeast to produce the highest volume and concentration of ethanol in yeast of 0,5 gram as much as 7,63 ml and ethanol content of 52,7%. Optimal fermentation time to produce the highest volume and concentration of ethanol in the fermentation time of 48 hours as much as 6,33 ml and ethanol content of 51,33%.