The radiative effects of cirrus clouds are evaluated based on observations extending over a period of six years (2000--2005) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) over the Tropics (25¿S--25¿N). The cloud radiative effect (CRE) shows a positive sign for optically thin cirrus (total-column cloud optical depth: τ 60%), their net effective CRE (net CRE ¿ cloud amount) is roughly twice as strong as that of thicker clouds; the modulation of the net radiative flux by variations in thin cirrus (particularly with 1 ≤ τ < 9) dominates-up to 15 W m-2-that by thicker clouds. These results suggest that thin cirrus has a significant effect on the tropical energy balance.