The present invention provides a process for preparing a bio-diesel, comprising, in the presence of an alkaline metal compound, reacting an oil-fat with C.sub.1-C.sub.6 monohydric alcohol in a reactor at a reaction temperature of from 130 to 280.degree. C. and a reaction pressure of from 1 to 12 MPa, separating fatty acid esters from the reacted materials, so as to produce the bio-diesel, wherein said alkaline metal compound is present in an amount of 0.001-0.07 wt %, in terms of the metal thereof, relative to the weight of the oil-fat. The process provided in the present invention has the advantages of great throughput and high yield of the bio-diesel.