A mobile terminal that can transmit voice data an picture, as well as a text message, in the form of a file during an instant messaging (IM) service, and its file transmission method are disclosed. When the IM service starts, a user inputs text to a message window to chat with one or more subscribers, and when voice or image data (picture, handwriting, image, etc.) is inputted while text is inputted, the inputted voice or image data is converted into a file, transmitted to a counterpart subscriber, and displayed in the form of an icon on a message window. The counterpart subscriber can check the voice or picture by selecting the displayed icon.