Owing to its low price and good shielding performance, concrete sees wide use as a radiation shielding material. In concrete structures, cracks occur due to thermal stress, hydration heat, weather, load and other reasons, and shielding performance changes according to the crack width. However, there are no design criteria providing the allowable crack size for shielding. This paper presents the design for a measuring instrument for quantifying the effect of cracks on shielding performance in a concrete body. The correlation between crack width and shielding performance is then deduced through experiments. A measuring system with a collimator is designed and fabricated and a concrete sample with collinear cracking is designed. The surface dose rate increased logarithmically according to the increase in crack width. The present experiment results are used to formulate an attenuation equation for gamma rays in a concrete structure with collinear cracks. The results of this study may be used for developing standards for radiological safety in shielding structures, especially in relation to the shielding margin. (C) 2007 Elsevier Ltd. All rights reserved.