Based on the well-known modified Angstrom formula on the relationship between the sunshine duration and the global solar radiation, this paper aimed to estimate the value of the constant a and b in Cameroon. Only five cities (Maroua, Garoua, NGaoundéré, Yaoundé and Douala) had the both available in-situ data recorded during the period of eleven years (1996-2006) beside which four others cities (Dschang, Koundja, Yoko and Manfé) had only the in-situ sunshine duration available data recorded during the period of twenty years (1986-2006). The 9 cities were grouped in 3 different climate regions. Based on the data of the 5 first cities belonging the 3 regions, the follow constant values a1 = -0.05, a2 = -0.02, a3 = -0.14 and b1 = 0.94, b2 = 0.74, b3 = 1.12 were obtained. The Root Mean Square Error (RMSE) Mean Bias Error (MBE) and correlation coefficient (r) were also determined. Then we used these values to estimate the global solar radiation for the other four remain cities. The constants a and b obtained values are in accordance with those of the West Africa region which Cameroon belongs to. So they can be employed in estimating global solar radiation of location in Cameroon paying attention only to the geographical location information.
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