An automatic gain control apparatus includes a first comparing module, a counting module, a second comparing module, and a control module. The first comparing module compares an input signal with a first threshold level and a second threshold level to generate a first compared output signal and a second compared output signal respectively. The counting module performs counting operations upon the first compared output signal and the second compared output signal to generate a first counting number and a second counting number respectively. The second comparing module compares the first counting number with a first threshold number to generate a first detection signal and compares the second counting number with a second threshold number to generate a second detection signal. The control module controls the gain of a radio frequency amplifier or an intermediate frequency amplifier according to the first detection signal and the second detection signal.