Abstract: Concrete structures are often subjected to conditions of different moisture contents, and the fracture toughness of concrete is affected significantly by its humidity. In this paper, we have conducted a systematic experiment study on concrete with different moisture contents. Concrete specimens C15, C20 and C30 were dried in an electric thermostatic drying oven until they had reached a completely dry state, and then they were immersed in water for 3, 10, 24, 48, 72, 120 and 240 h so as to achieve different moisture contents. Finally, three-point bending tests were conducted to measure their fracture toughness, and through analysis of the test data, prediction formulae for the relationship of fracture toughness versus moisture content were formulated. The results demonstrated an evident influence of concrete moisture content on fracture toughness, which took a significant decreasing trend with increasing moisture content and was decreased by 57% at saturated content from the value in completely dry condition. The P–COMD relation curves from the three-point bending tests can be divided into three stages, i.e. linear elastic deformation, steady crack extension, and unstable crack extension. As moisture content increased, the elastic stage gradually shortened, the steady extension stage lengthened, and the descending trend in the unstable stage became milder. Moreover, at a greater moisture content, the crack mouth opening displacement and the load-point deflection at the peak load condition were smaller, showing a lower deformability of concrete or its fracture toughness weakened.