A method for manufacturing liquid ejection heads includes the steps of forming ejection port members on a substrate, the ejection port members each having a liquid channel and an ejection port for ejecting liquid through the channel, the liquid channel communicating with the substrate; forming supply ports passing through the substrate to supply liquid to the channels; and forming a separation groove in the substrate to separate the substrate for each liquid ejection head. The step of forming the ejection port members includes the step of hardening a material constituting the ejection port member by heat treatment. The step of forming the separation groove is performed before the step of hardening.