An apparatus and method for operating a calculator mode in a portable terminal having a camera are provided. The apparatus and method comprise the steps of recognizing an arithmetic expression from an image taken by the camera and displaying the recognized arithmetic expression; shifting to an editing mode when an editing mode key is input, implementing editing operations, and editing the displayed arithmetic expression according to the editing operations; and implementing arithmetic calculation according to the edited arithmetic expression when a calculation implementation key is input and outputting a calculation result.