The atmospheric-pressure hydrodeoxygenation (HDO) of waste cooking oil (WCO) was investigated in a continuous fixed-bed reactor over a series of activated carbon (AC)-supported nickel phosphide catalysts with different initial Ni/P molar ratios (0.5-2.0) and nickel loading levels (1.16-38.90 mmol/g AC). The formation of the Ni2P phase on the AC, which was produced from commercial charcoal, as well as its structural and acidic properties was characterized by hydrogen-temperature programmed reduction (TPR), X-ray diffraction analysis, N-2 adsorption-desorption measurements performed at -196 degrees C, and ammonia-temperature programmed desorption. The effects of the Ni/P molar ratio, nickel loading level, reaction temperature, and gas hourly space velocity (GHSV) on the catalytic activity were elucidated. The complete formation of the Ni2P phase on the AC was observed at a Ni/P ratio of 1.5, while smaller Ni2P crystallite sizes were observed at lower Ni/P ratios. In addition, it was observed that the acidity increased and the specific surface area decreased with an increase in the nickel loading level, presumably because nickel phosphate is not readily reduced to Ni2P. The 5.37-Ni 2 P/1.5-TPR catalyst (Ni loading level of 5.37 mmol/g AC and Ni/P molar ratio of 1.5) exhibited good activity and stability during the HDO of WCO. The high-quality deoxygenated product primarily consisted of n-alkanes at the moderately high temperature of 300 degrees C and GHSV of 2.33 min(-1). Based on the results, we propose that the mechanism underlying the hydrotreatment of WCO involves hydrogenolysis, hydrodeoxygenation, dehydration-decarbonylation, and hydrogenation. To conclude, the synthesized Ni2P/AC catalyst could readily deoxygenate WCO at atmospheric pressure, producing n-paraffins as the primary component.