A method of manufacturing a monolithic inkjet printhead wherein the uniformity of the ink flow path is maintained by ensuring that the flow path forming layer and the nozzle layer are completely adhered to each other. The method includes forming a heater and electrode on a substrate, coating a negative photoresist on the substrate, and patterning the photoresist using a photolithography process to form an flow path forming layer that defines an ink flow path. The method further comprises steps for then forming a sacrificial layer so as to cover the flow path forming layer and then flattening upper surfaces of the flow path forming layer and the sacrificial layer using a chemical mechanical polishing (CMP) process such that when a nozzle layer is then formed, the flow path forming layer and the nozzle layer are completely adhered to each other.