A clock data calibration circuit including a first comparator, a multi-phase clock generator, a plurality of samplers, a plurality of data comparators and a data selector is provided. The first comparator compares first input data with second input data to generate a data signal. The multi-phase clock generator generates a plurality of clock signals, and the clock signals are divided into a plurality of clock groups. The sampler samples the data signal according to the clock groups to respectively generate a plurality of first sampled data signal groups. The data comparators respectively sample the first sampled data signal groups according to selected clocks to generate a plurality of second sampled data signal groups. Each data comparator generates a plurality of status flags according to a variation state of a plurality of second sampled data. The data selector generates a plurality of output data signals according to the status flags.