An eye width monitor (EWM) for a clock and data recovery (CDR) circuit includes a delay circuit, a first multiplexer (MUX) and a calibration circuit. The delay circuit includes an input terminal and an output terminal. The first MUX, coupled to the delay circuit, includes a first input terminal, a second input terminal and an output terminal. The first input terminal of the first MUX is coupled to a clock input terminal of the EWM. The second input terminal of the first MUX is coupled to the output terminal of the delay circuit. The output terminal of the first MUX is coupled to the input terminal of the delay circuit. The calibration circuit, coupled to the delay circuit, is configured to receive an oscillation clock from the delay circuit and receive a reference clock, and calibrate the oscillation clock with the reference clock.