A method of performing a preset operation by using voice recognition, the method comprising: performing the preset operation of a preset operation mode according to a key input or a touch input in the preset operation mode; and recognizing an input voice during performance of the preset operation of the preset operation mode and assisting the performance of the preset operation according to the recognized voice, wherein the key input or touch input for performing the preset operation is performed simultaneously with the voice input for assisting the performance of the preset operation, wherein the preset operation mode comprises a text writing mode, and performing the preset operation comprises displaying an input text according to the key input or the touch input in the text writing mode on a text display window, and assisting the preset operation comprises recognizing the input voice while displaying the input text according to the key input or the touch input on the text display window, comparing the displayed text with the recognized voice, determining that the displayed text has an error when the displayed text is not identical to the recognized voice, determining that the displayed text has no error when the displayed text is identical to the recognized voice, and converting the recognized voice to text and modifying the displayed text through a user's control when it is determined that the displayed text has the error, and when it is determined that the displayed text has no error, determining whether the recognized voice is a preset operation command; and performing the preset operation by applying the recognized voice when the recognized voice is the preset operation command. performing the preset operation of a preset operation mode according to a key input or a touch input in the preset operation mode; and recognizing an input voice during performance of the preset operation of the preset operation mode and assisting the performance of the preset operation according to the recognized voice, wherein the key input or touch input for performing the preset operation is performed simultaneously with the voice input for assisting the performance of the preset operation, wherein the preset operation mode comprises a text writing mode, and performing the preset operation comprises displaying an input text according to the key input or the touch input in the text writing mode on a text display window, and assisting the preset operation comprises recognizing the input voice while displaying the input text according to the key input or the touch input on the text display window, comparing the displayed text with the recognized voice, determining that the displayed text has an error when the displayed text is not identical to the recognized voice, determining that the displayed text has no error when the displayed text is identical to the recognized voice, and converting the recognized voice to text and modifying the displayed text through a user's control when it is determined that the displayed text has the error, and when it is determined that the displayed text has no error, determining whether the recognized voice is a preset operation command; and performing the preset operation by applying the recognized voice when the recognized voice is the preset operation command.