A method for determining a power-save mode of multimedia application, when a user operates a computer system through a peripheral input device, an image sensor retrieves a first image data corresponding to the user's facial feature. Next, the first image data is stored into the computer system. Then, the computer system confirms whether an input signal from the peripheral input device is received or not within a preset time. Once the preset time is reached, the image sensor is used to retrieve a second image data. Then, the computer system compares the second image data with the first image data, and if the matching rate between the second image data and the first image data falls out of a specific range, the computer system enters the power-save mode.