1. A method of operating a security function of an electronic device that is configured for wireless communication, the method comprising: receiving a user request for activation of a security function during booting of the electronic device in a non-trusted operation environment; confirming whether a security indicator, which is editable during the booting of the electronic device for changing at least one parameter that defines the security indicator, is set; and when the security indicator is set, initializing an operating system configured for use with a trusted operation environment, applying the security indicator to a security function screen image, in response to the user request for the activation of the security function, and outputting the security function screen image to a display of the electronic device.