Biodiesel obtained from the transesterification of vegetable oil or animal fat is a promising renewable green alternative fuel for compression ignition engines. Compression ignition engines are particularly suitable for medium-to-large road, rail and marine use. This is due to their excellent efficiency and longer operation life which is about twice as much as that for spark ignition engines. The replacement of conventional diesel fuel with biodiesel fuel is an attractive solution since the latter is regarded as a renewable, biodegradable, non-poisonous, and oxygenated fuel. However, existing production technologies offer less competitive prices than petroleum-derived diesel due to high input feed and biodiesel purification costs. Non-edible vegetable oils such as waste cooking oils may be used as cheaper substitutes to virgin edible vegetable oils in the feed stream. Similarly, the utilization of solid heterogeneous catalysts in biodiesel synthesis instead of conventional homogeneous catalysts could diminish the complexity of biodiesel processing. This is because they can be easily separated from the reaction mixture and hence, improved product purity. Therefore, the current study involves the production of biodiesel fuel from waste cooking oil with ethanol in presence of tri-potassium phosphate (K₃PO₄) as a solid base heterogeneous catalyst. The principal variables for the present study were, ethanol/oil molar ratio (6:1 to 12:1), catalyst concentration (2-8 wt %), stirring speed (600-1800 r/min), reaction temperature (30-70°C) and time (1-3 hours).Runs were conducted following a 5-variable central composite design of experiments in order to secure the optimal set of variables combination. Liquid phase composition was determined from GC-mass analysis. The effect of the operating variables on the presence of different fatty acid ethyl esters was evaluated. Fuel properties including iodine and saponification values were practically calculated for each produced biodiesel sample. Higher heating value and cetane number were also measured. Thus, the effect of five examined operating variables on fuel properties was also evaluated.Results show that ethanol/oil molar ratio and catalyst concentration posed the most positive impacts on ethyl ester yield as well as all the evaluated fuel properties. However, the influence of catalyst concentration was found to be more obvious. Reaction temperature displayed a slightly lower effect, while reaction time depicted the least influence. In contrast, stirring speed showed a quite negative effect on ethyl ester yield. Optimal ester yield of (95.2%) was achieved at 12/1 ethanol/oil molar ratio, 8 wt% catalyst concentration, 600 rpm stirring speed, 70°C reaction temperature after one hour reaction. Similarly, these conditions were considered as the optimal route that produces best fuel quality in terms of higher heating value and cetane number.