The fluorescence intensity of a two-level atom system driven by a two-monochromatic field has been calculated. The results show that the intensity oscillates with time in the long-time limit and the oscillation is the superposition of a series of harmonic waves. The case for maximum oscillation amplitude modulation is reached when one near-resonant monochromatic field acts with an atom, while the difference in frequency of the two monochromatic fields is equal to the Rabi frequency for the first monoehromatic fields is equal to the Rabi frequency for the lirst monochromatic field. We call this phenomenon quanturn beat in resonance fluorescence.