A level transition determination circuit includes a multi-phase clock generator, an oversampling unit, and a state detection circuit. The multi-phase clock generator is used for receiving an input clock signal and generating S×N clock signals, in which S and N are integers. Each clock signal is synchronized to the input clock signal and has a different delay time. The oversampling unit is used for performing N-times oversampling on M bit periods of the serial input data according to the clock signals, so as to generate M×N sampled values in parallel during the M bit periods. The state detection circuit is used for receiving (M×N)+1 sampled values and generating N detection signals by detecting level transitions between adjacent sampled values of the (M×N)+1 sampled values and the level transition results.