The invention discloses a mode detection circuit and a method thereof, for detecting an image signal, the image signal includes a horizontal resolution and the vertical resolution. The mode detection circuit includes a measuring unit, a calculation unit, and a decision unit. The measuring unit receives a clock signal and is used to count the clock signal to output a first counting value and the second counting value. The calculation unit is used to perform the calculation with the first counting value and the second counting value and thereby outputting a calculating value, wherein the calculating value outputted by the calculation unit is corresponding to the ratio of the first counting value to the second counting value. The decision unit is used to determine the horizontal resolution or the vertical resolution according to the calculating value.