Drying is simply the process of moisture removal from a product. It call be performed by various methods. In these methods, thermal drying is most commonly used for drying agricultural products. These drying systems are usually classified into low and high temperature dryers according to their operating temperature ranges. In low temperature drying systems, natural-circulation greenhouse-type dryers appear the most attractive option for use in rural locations. They are superior operationally and competitive economically to natural open sun drying. Therefore, two different types of natural-circulation greenhouse crop dryers were designed, constructed and tested for their performance in this investigation. Each dryer mainly consisted of a framework constructed from black coated metal bars, corrosion-resistant plastic mesh and a black coated solar radiation absorber surface. The frameworks of the dryers were clad with clear polyethlene sheet on the all sides. The cladding at rear side was arranged to allow put the moist products into the drying chamber or get dried product from there. The clear plastic cladding at the bottom edge of the front side and rear side was also arranged to allow air to flow into the chamber, while the rectangular stream at the top of the end served as the exit for the moist exhaust air. The dryers were tested in the summer conditions. They were aligned lengthwise in the north-south axis during experiments. All dryers were experimented without crops (no product loaded) and with crops (pepper loaded). The dryers were also tested with chimney constructed from a galvanised iron sheet and without chimney in order to determine the effect of the chimney on the air flows. In addition, pepper was dried in the opell-sun drying in order to compare the greenhouse dryers with open air drying. During experiments, temperature distribution inside drying chamber, pepper surface temperature, moist exaust air velocity, wind velocity, solar radiation, temperature and relative humidity of the atmosphere air weremeasured. Pepper samples, collected from open sun dryer and different positions in greenhouse dryers were also weighted to note amount of water evaporated. The results of the study show that the use of natural-circulation greenhouse dryers for drying agricultural products, is 2...5 times more efficient than open air drying and using black coated solar radiation absorber surface and chimney improve the performance of these dryers. Besides, the output of the greenhouse type dryers has also high quality, compared with open air drying. (c) 2005 Elsevier Ltd. All rights reserved.