In the design of flexural strength, the strength reduction factor phi decreases from tension-controlled sections to compression-controlled sections to increase safety with decreasing ductility. This paper presents how to determine the reduction factor for flexural strength of reinforced concrete beams according to ACI code. In the reliability-based design, the reliable prediction of the flexural strength of reinforced concrete members is assured by the use of reduction factors corresponding to different target reliability index beta. In this study, for different beta and coefficients of variation of the flexural strength parameters, the flexural strength reduction factor has been investigated by using experimental studies available in the literature. In the reliability analysis part of the study, the first-order second moment approach (FOSM) has been used to determine the reduction factor. It has also been assumed that the random variables are statistically independent.