A method of manufacturing a window member includes performing a first reinforcement operation including performing a first ion-exchange treatment on an initial window member. The first ion-exchange treatment includes applying ion salts at a temperature equal to or greater than a first temperature of about 500° C. A stress relief operation includes performing a heat treatment and/or a salt treatment on the initial window member to which the first reinforcement operation is performed. A second reinforcement operation includes performing a second ion-exchange treatment on the initial window member to which the stress relief operation is performed.