In order to lower the high prices of individual feedstock, extend the life of a more limited feedstock and to improve the fuel properties of biodiesel it imperative to optimize process parameters for biodiesel derived from mixed feedstocks. Samples of castor seeds were collected from Jimma zone, southwest Ethiopia while sample of wet microalgal biomass was obtained from wastewater stabilization ponds using a 60µm filter screen. The castor seeds and algal biomass were sun-dried before further dried at 80 0 C in an oven and ground to pastes. Oils were extracted from dried and milled castor seeds and micro-algae pastes with a Soxhlet apparatus using methanol. The extracted oil was purified and characterized before converted to biodiesel. A transesterification process designed using Response Surface Methodology (RSM) based on central composite design (CCD) experimental design was used to optimize the biodiesel production process parameters from mixed oil using alkaline catalyst. The Design Expert® 12 software was used to analysis experimental results. The effects of catalyst concentration, ethanol to mixed oil molar ratio and reaction temperature on the biodiesel yield were investigated using the experimental results. Accordingly, the optimum conditions for biodiesel production from mixed oil were a catalyst concentration of 1.23% w.t of the oil, alcohol to mixed oil molar ratio of 5.94 :1 (v/v) and reaction temperature of 51.30 0 C. The yield of biodiesel under these conditions was 93.88% . Experiment was conducted under the specified optimum conditions to validate the result predicted by the software. The yield of biodiesel from the experiment was 93.36 % which is very close to the value predicted by the software. The fatty acid composition of the biodiesel from mixed oil was analyzed using Gas chromatograph. The various fuel properties of biodiesel were determined using standard methods and results were compared with ASTM D6751 and EN 14214 standards. The physicochemical properties fulfill both standards.