A method of manufacturing a monolithic inkjet printhead. The method may include forming on a substrate a heater for heating ink and an electrode for supplying current to the heater, forming a passage forming layer that surrounds an ink passage by applying negative-type photoresist to the substrate and patterning the same, forming a sacrificial layer having a planarized top surface in a space surrounded by the passage forming layer by repeatedly applying a positive-type photoresist to the substrate having the passage forming layer and patterning the same by photolithography at least twice, forming a nozzle layer having a nozzle by applying a negative-type photoresist to the passage forming layer and the sacrificial layer and patterning the same, etching the substrate from the bottom surface thereof to be perforated and forming an ink supply hole, and removing the sacrificial layer. Since the top surface of the sacrificial layer is planarized, the shape and dimension of the ink passage can be easily controlled, thereby improving uniformity of the ink passage.