Background & Objective: Recommended illumination Engineering Society of North America (IESNA) methods were used to measure the illumination in regular geometric areas such as square or rectangular form areas. It seems that with using the GIS method which is used to save, manage and spatial data analysis and processing them by computer we would be able to examine the illumination in many situations and make new predictions. Materials & Methods: In this research the Testo 545 luxmeter sensor was put in height of 30 inches (76 cm) from the bottom of irregular geometric study saloon in libraries of Zanjan. Then total, natural and artificial illumination was measured in the center of each station in lux. These data were entered into Arcview software as input data for analyzing. Thus the illumination quantities, the area coverage percent and isolux curves were obtained for each station. Results: The results showed that illumination quantities in the libraries studied were not suitable because, total, natural and artificial illumination in 51% , 80% and 99% of the libraries were less than 300 lux (IESNA recommended) respectively. Therefore it is necessary to redesign the artificial lighting systems or optimize them in order to achieve the standard illumination. Conclusion: It seems that GIS method could show the distribution of the illumination in the irregular geometric library halls. These data can also be showed as isolux curves with use of interpolation technique by ArcView software.