A smart card includes an internal voltage generator, a clock generator, and an internal circuit. The internal voltage generator generates a first internal voltage and a second internal voltage based on an input voltage received through an antenna. A level of the second internal voltage is lower than a level of the first internal voltage. The clock generator receives the first internal voltage and the second internal voltage to generate a clock signal. A frequency of the clock signal is changed according to the level of the first internal voltage. The internal circuit operates based on the clock signal and the second internal voltage.