Currently the supply of fossil energy is running low; besides, it contributes very significantly to environmental pollution and climate change. Therefore, it is necessary to develop new and renewable alternative energy, one of which is bioethanol. One of the potential raw materials used in bioethanol production is sugarcane bagasse which is an abundance of agricultural waste. This study aims to determine the concentration of Na-alginate and the density of cell suspension with the best activity and stability of immobilized Saccharomyces cerevisiae ATCC 9763 cells in the bioethanol production process. This research is an experimental study using a randomized block design (RBD) with a factorial pattern consisting of two factors, namely the first factor is the concentration of Na-alginate and the second factor is the concentration of cells suspension in making immobilized S. cerevisiae cells. The first factor consists of 5 levels, namely 0%, 2%, 3%, 4%, and 5% (w/v). The second factor consisted of 3 levels, namely cells with OD660 20, 25, and 30. The parameters observed were the activity of immobilized S. cerevisiae cells including several variables, namely reducing sugar content, pH, and ethanol content. The stability of immobilized S. cerevisiae cells was seen from the level of cell turbidity (OD660 nm). The results showed that the concentration of Na-alginate, the concentration of S. cerevisiae cells, and the interaction between treatments had a very significant effect on the activity and stability of immobilized S. cerevisiae cells. The treatment of Na-alginate 2% (w/v) with cell density 25 was able to produce ethanol with a higher level of 63.87 ppm compared to treatment with 0% Na-alginate (free cells) with the level of cell density 25 was 56.97 ppm.