Designing, Modeling, and performance analysis of solar energy systems, depend on solar radiation data. Accurate data for many cities and locations in Iran is not available. Therefore, estimation of such data for solar applications is inevitable. Clearness index (KT) is a measure of solar radiation attenuation at a location due to geographic-atmospheric parameters. In this paper, a model for prediction of clearness index of many cities of Iran is presented. In this model KT is assumed to be a function of sunshine duration, relative humidity, precipitation, and ambient temperature. The model parameters are adjusted based on available data for a few locations, then it is used to predict KT for other cities. The model predicts the data favorably and its results are compared with available data and prediction other models. Among The cities investigated, Yazd with 0.648 has the highest KT and Ramsar with 0.383 the lowest one.