Early blight disease caused by Alternaria solani spreads well in abundance of humidity and the presence of water on the leaves. The early symptoms of disease are brown to black spots as concentric embowed rings. The fungal spores are distributed by wind and rain to healthy leaves. High humidity and temperatures between 18 and 25 °C, dew, rain and sprinkler irrigation increase its spread. The purpose of this study was to study the influence of meteorological variables (Temperature, Relative humidity and Precipitation) on spores’ population of early blight pathogen of potato and possible prognosis of disease in Kermanshah province during 2013-2014 crop seasons. To perform the study, potato plants were cultivated in Razi University farm and the symptoms of the disease were monitored continuously through the season. Besides, the number of fungi spores were also measured using a standard spore trap. On the other hand, two early blight prognosis empirical models were developed using on-site meteorological data and those obtained from nearby synoptic station. The correlation between the population of spores in the air and meteorological variables were worked out. The results showed that there exist a significant relation between the spore population and maximum-minimum air temperature (p= 0.01) and relative humidity (p=0.05). According to results of this study, the proposed model can be satisfactorily used for early blight early warning in western region of Iran.