An oil-based ink for a printing plate by an ink jet process comprising spraying an oil-based ink comprising at least 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 a water-resistant support having a lithographically printable hydrophilic surface to dropwise from a nozzle form an image, wherein the resin particles dispersed are polymer particles obtained by polymerization granulation of a solution comprising (i), (ii) and (iii): (i) at least one monofunctional monomer (A) which is soluble in a nonaqueous solvent at least miscible with the nonaqueous carrier liquid and becomes insoluble therein by polymerization; (ii) (ii-a) at least one monomer (B) represented having a specific substituent copolymerizable with the monomer (A), or (ii-b) at least one specific monofunctional macromonomer (MA) having a weight average molecular weight of 2.times.10.sup.4 or less; and (iii) at least one dispersion-stabilizing resin soluble in the nonaqueous solvent and having no graft group polymerizing with a monomer, and a method for preparing a printing plate by an ink jet process using the ink.