A decision feedback equalizer includes: a feedforward equalizer, a feedback equalizer, a slicer and a decision adjustment unit. The feedforward equalizer is arranged to generate a feedforward output signal based on an input signal. The feedback equalizer is coupled to the feedforward equalizer and arranged to generate a feedback output signal according to a decision output signal. The slicer is coupled to the feedforward equalizer and the feedback equalizer, and is controllable by a decision adjustment parameter, wherein the slicer is arranged to perform a slicer decision on a sum of the feedforward output signal and the feedback output signal, thereby generating the decision output signal. The decision adjustment unit is coupled to the slicer, and is arranged to adjust the decision adjustment parameter according to a sleep state of a communication device in which the decision feedback equalizer is disposed.