We investigate a novel quantum effect that an atom coupled to a monochromatic radiation field may exhibit. The quantum effect has to do with the damping rate of the amplitude of the Rabi oscillation when the atom is subjected to spontaneous decay. As is well known, the amplitude of the Rabi oscillation decreases progressively at each successive revival, even in the absence of spontaneous decay and any other dissipation, because a smaller and smaller number of dephased oscillations rephase at each successive revival time. Spontaneous decay works to further decrease the oscillation amplitude over the already existing decrease in its absence. We are concerned with this additional damping due to spontaneous emission. We show that the semi-classical theory tends to overestimate this damping rate, and thus a correct quantitative description of the damped Rabi oscillation is obtained only with a full quantum mechanical treatment.