A method of manufacturing an inkjet printhead includes forming a heater and an electrode on a substrate, forming a flow path forming layer by coating a first negative photoresist composition on the substrate, forming a sacrifice layer, planarizing the flow path forming layer and the sacrifice layer, forming a nozzle layer by coating a second negative photoresist composition on the flow path forming layer, forming an ink feed hole in the substrate, and eliminating the sacrifice layer, wherein the first and second negative photoresist compositions include a prepolymer which comprises one selected from the group consisting of a glycidyl ether functional group, a ring-opened glycidyl ether functional group, and an oxytein functional group in a monomer repeat unit, and one selected from the group consisting of a phenol novolac resin-based backbone, a bisphenol-A-based backbone, a bisphenol-F-based backbone, and an alicyclic backbone; a cationic initiator; a solvent; and a plasticizer.