A control method of an alterable key is applied in a portable electronic device including switch a plurality of function modes. The portable electronic device has an opaque key portion, a pattern template layer, and a plurality of light emitting portions. A pattern template layer has many groups of operating patterns. Light emitting portions are corresponding to the operating patterns and emit the specific light, and the operating patterns are corresponding to the function modes. The control method includes the following steps of receiving a first command to switch the corresponding function mode, brightening the light emitting portion corresponding to the function mode to show the corresponding operating patterns on the opaque key portion, receiving a second command to switch another corresponding function mode, and brightening the light emitting portion corresponding to another function mode to show another corresponding operating patterns on the opaque key portion.