Abstract: In order to obtain the global solar radiation for practical applications, a mathematical model was developed in this study. The experimental data for five years were correlated in order to calculate the model parameters. Knowing the values of cloudiness degree k, Julian date N, the global solar radiation can be estimated at any hour t by the developed model. We have testing this model at different periods of two years and at different cloudiness degree classes, in the second a monthly global solar radiation was estimated and compared with the results of Sivkov model. To evaluate the accuracy of the model, three methods are used, RMSE, MPE and MBE. The mean monthly and yearly global radiation was estimated with an accuracy of 4.5 and 0.34%, respectively. The prediction results were found to be in good agreement with the experimental data can be employed for estimating global solar radiation for the south of Tunisia.