We developed a simple physical model of Venus' cloud to understand the influence of cloud on the evolution of the surface environment and the climate on Venus. For a given atmospheric structure and composition, the cloud structure and its albedo are calculated through our model. We successfully reproduced the present Venus albedo: the calculated value is 0.87 for 550-nm wavelength, while the observed one is 0.85. The variability of cloud albedo caused by the change of H2O abundance in the Venus atmosphere is examined. Although H2O is merely a minor component of the atmosphere, its influence on the surface environment is found to be quite large. Increase in H2O abundance raises the albedo and cools the surface environment. When H2O abundance is smaller than a critical value of about 0.5 ppmv, complete evaporation of cloud occurs owing to a warming of the atmosphere. When H2O abundance is increased, the H2O abundance in the upper atmosphere is reduced owing to a lower atmospheric temperature produced by high albedo of thick clouds. The cooling of the atmosphere will affect the escape flux of hydrogen from the Venus atmosphere into space. ¿ 2001 American Geophysical Union