Semiconductor memory device and operation method thereof includes an output enable signal generator configured to synchronize a read command to a data clock signal to generate an output enable signal according to a CAS latency, a sampling control signal generator configured to generate a sampling control signal that is activated during a period corresponding to an activation timing of the output enable signal and an end timing of data output, a read clock signal generator configured to sample the data clock signal in response to the sampling control signal to generate a read clock signal and a data output circuit configured to output data according to the read clock signal.