The quantity of solar radiation received by the earth's surface is very important to numerous renewable energy applications. However, direct measurement of solar data is not widely available, especially in developing countries. This paper uses Particle Swarm Optimization (PSO) to train an artificial neural network (PSO-ANN) using data from available measurement stations to estimate monthly mean daily Global Solar Radiation (GSR) at locations where no measurement stations are available. The inputs to the networks are: month of the year, latitude, longitude, altitude, and sunshine duration, and the output is the monthly mean daily GSR at the specified location. Using training data from 31 stations and testing data from 10 locations, the PSO-ANN outperforms a neural network trained using the standard backpropagation (BP) algorithm (BP-ANN) with an average Mean Absolute Percentage Error (MAPE) of 8.85% for the PSO-ANN and 12.61% for the BP-ANN. The performance is improved significantly, when we use the leave-one-out method, where data from 40 locations is used for training and data from the 41st station is used for assessing the performance. In this case the average of MAPE on data from the 10 testing stations is about 7%. We used the same method to assess the performance of the PSO-ANN on testing data from each of the 41 stations with an overall average MAPE of about 10.3%. Comparison with BP-ANN and an empirical model showed the superiority of the PSO-ANN. (c) 2012 Elsevier Ltd. All rights reserved.