Abstract The performance of a thermal collector can be evaluated by determination of thermal efficiency in steady state test of solar simulators. This paper presents design and construction of an efficient multiple-lamp solar simulator for investigating the performance of the solar collectors for scientific and industrial purposes. Metal halid lamps are employed as source of irradiance. The uniformity of the irradiated surface was checked by simulation of the light field. This simulations are done in DIALux. The non-uniformity and instability, as factors that are described in EN 12975 standard, must be less than 15%. Also, the standard is determined, at least, 700 W/m 2 as average of irradiance for testing solar collectors. The amount of irradiance received by their radiated surface, which can vary from 100 W/m 2 to 1000 W/m 2 , is controlled by changing the distance of testing surface from lamps and/or the number of lamps that is on. The performance of solar simulator was compared to Sun by testing a Compound Parabolic Concentrating (CPC) collector at Materials and Energy Research Center (MERC) in the city of Karaj. In the same conditions of test, the thermal efficiency of CPC collector was calculated with both sources of radiation from the solar simulator and Sun.