An eye pattern generator for generating an eye pattern of an input signal is provided. The eye pattern generator includes first and second comparators and a control circuit. The first comparator receives the input signal, a clock signal, and a first voltage and compares the input signal with the first voltage according to the clock signal to generate a first comparison signal. The second comparator receives the input signal, the clock signal, and a second voltage lower than the first voltage and compares the input signal with the second voltage according to the clock signal to generate a second comparison signal. The control circuit changes at least one of a level of the first voltage and a level of the second voltage according to the first and second comparison signals to form a region boundary between an open-eye region and a closed-eye region of the eye pattern.