This paper presents a new expression to describe the flexural fatigue strength of plain concrete. It incorporates the effects of stress ratio and stress level into the faitgue equation and allows a statistical description of fatigue life in terms of both stress level and stress ratio. A concept of equivalent fatigue life is introduced in the derivation of the new expression. Laboratory flexural fatigue experiments were carried out on 78 plain concrete beams to verify the validity of the fatigue equation. The result shows that the proposed fatigue equation could model the laboratory fatigue data of different stress levels and stress ratios with statistical correlation values exceeding 0.90, and the distribution of the equivalent fatigue life is in excellent agreemtent with a 2-parameter Weibul distribution.