A method of manufacturing an inkjet printhead includes preparing a substrate having a heater to hear ink and an electrode to supply current to the heater, applying a crosslinked polymer resist composition to the substrate having the heater and the electrode and patterning the same, and forming a passage forming layer that surrounds an ink passage, patterning the substrate having the passage forming layer by photolithography at least twice, and forming a sacrificial layer having a planarized top surface in a space surrounded by the passage forming layer, applying the crosslinked polymer resist composition to the passage forming layer and the sacrificial layer and patterning the same, and forming a nozzle layer having a nozzle, etching the substrate from the bottom surface thereof to be perforated, and forming an ink supply hole, and removing the sacrificial layer, wherein the crosslinked polymer resist composition comprises a precursor polymer that is a phenolic novolak resin having glycidyl ether functional groups on repeating monomer units.