The Schmidt hammer has been commonly used device for hardness determination and for predicting the unconfined compressive strength and other mechanical properties of rocks, due to the fact that it is a quick, easy, inexpensive and non-destructive testing method. Testing is most commonly performed following the ISRM and ASTM standards. The effect of sample size for a consistent hardness value has not been well defined in previous works. ISRM [ Rock characterization testing and monitoring ISRM suggested methods, suggested methods for determining hardness and abrasiveness of rocks, Part 3. Oxford: Pergamon; 1981. p. 101-3] suggested that block edge length should have at least 6 cm, while ASTM [ Standard test method for determination of rock hardness by Rebound Hammer Method, D5873-05, 2005] indicated at least 15 cm. In this study, in order to analyze the effect of sample size on Schmidt rebound hardness (SRH) property of rocks, rock samples were collected from eight locations. Cubic samples having different edge dimensions of 6, 7, 8, 10, 12 and 15 cm were prepared. In the laboratory, Schmidt hammer tests were conducted according to the suggested procedure by ISRM ( 1981) together with different methods of recording SRH. By evaluating the measured data, it is showed that the size of the cubic samples significantly affects the SRH values. Edge dimension of the cubic block should be at least 11 cm for determining a consistent hardness value. An equation predicting the consistent hardness value from samples smaller than 11 cm was also suggested. (C) 2008 Elsevier Ltd. All rights reserved.