The regression coefficient of the well-known Angstrom correlation are determined for Riyadh city (the capital of Saudi Arabia) longitude 46 degrees 14＇6 24 degrees 55＇N The relationship of the daily and monthly variation of the fraction of the diffuse solar irradiation to extrateresstrial H-d/H-o=K-d and the clearness index H/H-o=K-T are obtained. The variation of the values of the average daily global solar radiation against the month of the year is reported. The value of K-n defined as the ratio of direct normal Insolation, H-bn measured in Riyadh to direct normal solar extraterrestrial radiation with the K-T and K-d were obtained. The daily diffused ratio H-d/H and the daily clearness index are shown as a function of the month of the year.