A scaler includes an input interface, an output Vsync pulse generating circuit and a data buffer circuit. The input interface is arranged to receive an input Vsync pulse and input image data. The output Vsync pulse generating circuit is arranged to accordingly generate a first output Vsync pulse and a first output request in response to the input Vsync pulse. The data buffer circuit is arranged to buffer the input image data and, in response to the first output request, output a first output frame according to the input image data. The output Vsync pulse generating circuit further generates a second output Vsync pulse and a second output request according to the first output Vsync pulse and a first predetermined period and in response to the second output request, the data buffer circuit further outputs a second output frame according to the input image data.