An oil-based ink for the preparation of a printing plate by an ink jet process comprising dropwise Supplying an oil-based ink comprising resin particles dispersed in a nonaqueous carrier liquid having an electric resistance of 10.sup.9 .OMEGA.cm or more and a dielectric constant of 3.5 or less on an image-receiving layer of a lithographic printing plate precursor comprising a water-resistant support and the image-receiving layer provided thereon which contains zinc oxide and a binder resin and whose surface has a contact angle with water of 30 degrees or more to form an image and subjecting the non-image area of the image-receiving layer to oil-desensitization upon a chemical reaction treatment to prepare a lithographic printing plate wherein the resin particles dispersed are copolymer resin particles obtained by polymerization granulation of a solution of at least one monofunctional monomer, at least one monofunctional macromonomer and at least one resin for dispersion stabilization.