The major part of all energy consumed worldwide comes from fossil sources (petroleum, coal and natural gas). However, these sources are limited, and will be exhausted by the near future. Biodiesel (Methyl esters of fatty acids), an alternative diesel fuel, is made from renewable biological sources such as vegetable oils and animal fats. This fuel is biodegradable and non-toxic and has low emission profiles as compared to petroleum diesel. Usage of biodiesel will allow a balance to be sought between agriculture, economic development and the environment. Recently, the demand of biodiesel has increased due to petroleum price rises in the last few months and the development of government measures like the EU Directive 2003/30/EC on the promotion of the use of biofuels or other renewable fuels for transport. Methyl esters of fatty acids are usually produced by transesterification of triglyceride to methyl esters using sodium or potassium hydroxide dissolved in methanol as homogeneous catalysts; however, in this conventional method removal of these catalysts is technically difficult and a large amount of waste water was produced to separate and clean the catalyst and the products. Therefore, for the development of an environmentally benign process and the reduction of the production cost, a new process using heterogeneous catalyst should be introduced. Heterogeneous catalysts could improve the synthesis methods by eliminating the additional processing costs associated with homogeneous catalysts and minimizing the production of pollutants. This study deals with the transesterification of cotton seed oil with methanol and has been studied in a heterogeneous system, using Zinc oxide loaded with potassium fluoride as solid base catalyst. Influence of catalyst preparation conditions as well as the effects of various reaction variables such as the catalyst loading, oil to methanol ratio, reaction time, agitation speed on the conversion of cottonseed oil and used frying oil were investigated. All transesterification reactions were carried out at 80 °C and pressure 55 psi. According to the experimental results Zinc oxide loaded with potassium fluoride was demonstrated to be a strong solid-base catalyst for the transesterification of vegetable oils with methanol. The catalyst with 20 wt.% KF loaded on ZnO and after calcinated at 600 °C for 7 h was found to be the optimum catalyst, which can give the best catalytic activity. When the transesterification reaction of cotton seed oil was carried out at temperature 90 °C, with a molar ratio of methanol to oil of 10:1, a reaction time 2.5 h, a catalyst amount 2.5 % and a agitation speed of 600 rpm, the conversion of cottonseed oil was 97.3%. All of the measured properties of the produced biodiesels met the current quality requirements according to EN 14214.