It is important to consider the effect of member size when estimating the ultimate strength of a concrete flexural member, because the strength always decreases with an increase of member size except for well-reinforced members. Research conducted previously in this area include axial compressive strength size effect on cylindrical specimens and flexural compressive strength size effect on C-shaped specimens, notched cylindrical specimens, and axially loaded double cantilever beam (DCB) specimens. Since the most widely used flexural member type is reinforced concrete (RC) beams, it is logical to extend the study of flexural compressive strength size effect to flexural loaded RC beam members. Previously, several researchers have reported from their studies that flexural compressive strength size effect does not exist. However, the analyses show that the specimens used for the study had limited size variation and the neutral axis depth variations were too similar to show distinct size effect. Therefore, this study enforced distinct neutral axis depth variations for all of the tested specimens. In this study, the size effect of a RC beam was experimentally investigated. For this purpose, a series of beam specimens subjected to four-point loading was tested. RC beams with three different effective depths were tested to investigate the size effect. The shear-span to depth ratio and the thickness of the specimens were kept constant to eliminate the out-of-plane size effect. The test results are curve fitted using Levenberg-Marquardt's Least Square Method (LSM) to obtain parameters for Modified Size Effect Law (MSEL) by Kim et al. The analysis results show that the flexural compression strength and ultimate strain decrease as the specimen size increases. Comparisons with existing research results considering the depth of neutral axis were also performed. They also show that the current strength criteria-based design practice should be reviewed to include member size effect. (C) 2006 Elsevier Ltd, All rights reserved.