Global warming, and consequently climate change are important topics studied extensively by researchers throughout the world in the recent decades where changes in climatic parameters are investigated. Considering large-scaled output of AOGCMs and low precision in computational cells, uncertainty analysis is one of the principles in hydrological studies. For this reason, the uncertainty due to precision of computational cells and in passing from global scale to regional scale through LARSWG model and CRU institute, precipitation changes in Mashhad synoptic station located in Ghareghom basin were analyzed. The results showed enough ability of the model to simulate precipitation parameter in the base period. Downscaled output of HadCM3 generated by CRU with high precision shows gradual decreasing of precipitation trend for frequency and sum values. Comparing the downscaled output of the AOGCM with 2.5*3.75 resolution and the output of CRU with 0.5*0.5 resolution, the uncertainty is due to precision of computational cells from global to regional scale: the latter scale is closer to real values.

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