A random bit generator and a method thereof are provided. The random bit generator includes an amplifier, a comparing circuit, an oscillator, a sampler, and a storage circuit. The amplifier amplifies a difference between input signals generated based on thermal noise. The comparing circuit compares an alternating current (AC) signal output from the amplifier with a direct current (DC) signal obtained by low-pass filtering the AC signal and outputs a signal according to the comparison result. The oscillator may be implemented with a resistor-capacitor (R-C) oscillator. The oscillator consumes lower current and occupies a smaller layout area than a voltage controlled oscillator (VCO) and outputs an oscillation signal. The sampler samples the oscillation signal output from the oscillator in response to the output signal of the comparing circuit. The storage circuit stores an output signal of the sampler in response to a clock signal.