Increased global demand for energy and fossil fuels in particular as a result of the noticeable increase in industrialization and modernization in the world However, the energy derived from this source is considered a major source of pollution. This prompted the hunt for a different energy source that produced from renewable, non-polluting raw materials, such as biodiesel, as an alternative, clean, and non-polluting fuel. Castor and Rapeseed seeds oil are considered waste in Egypt because their production exceeds the requirements of agriculture and industry. Therefore, they are considered an important and available source to produce biodiesel from their oils, and therefore they are sustainable, abundant, and cost-effective. In this paper, three primary parameters were examined in relation to the synthesis of biodiesel from each oil at 1% catalyst concentration: reaction time, reaction temperature, and methanol concentration. To speed up the conversion of free fatty acids (FFA), methanol potassium hydroxide (KOH) was added to the transesterification process as a homogenous alkaline catalyst. Exists in the oil to fatty acid methyl ester (FAME). The highest biodiesel mass yield for castor seed oil was 87.96 at the optimum condition: 20 wt.% methanol 90°C for 60 minutes. In addition, the highest biodiesel mass yield for rapeseed oil was 89.405% at the optimum condition: 20 wt.% methanol, 90°C reaction temperature for 45 minutes. A comparison study was made between ASTM biodiesel standard, commercial.