An ink jet-recording ink capable of being cured through the irradiation with a radioactive ray, in which the ink jet-recording ink includes a coloring material, a polymerizable compound and a polymerization initiator system, wherein the polymerization initiator system includes a polymerization initiator and at least one co-sensitizing agent selected from the group consisting of compounds represented by the foregoing general formulas (i) to (iv) and (vii) to (ix), as well as a method for preparing a lithographic printing plate includes the steps of (1) A step for injecting an ink jet-recording ink as set forth in claim 1 on the surface of the substrate; and (2) A step for curing the ink by the irradiation of the substrate carrying the injected ink thereon with a radioactive ray to thus form an image of cured ink on the substrate. The ink is highly sensitive to a radioactive ray, permits the formation of a high quality image and has excellent storage stability. In addition, the method permits the formation of a high quality image excellent in the printing durability.