<p>With the growing use of precast concrete structures in construction, the reliability-based design (RBD) of bending capabilities of precast reinforced concrete (RC) and prestressed reinforced concrete (PRC) beams has become increasingly important. Traditional RBD methods, such as the partial safety factor design (PSFD), suffer from limited precast beam data for calibration and fail to consider the impact of different failure modes on flexural capacity. This oversight can lead to an unreliable estimated flexural design capacity. Hence, a robust RBD approach must account for the unique features of precast beams, including limited sample sizes and failure modes. In this study, a probabilistic failure mode-based design model for evaluating the flexural capacities of precast RC/PRC beams was developed and integrated with the probabilistic confidence-based estimation (PCBE) RBD method, which is suitable for small samples. This combined approach enables reliable estimation of flexural design capacities of precast RC/PRC beams. The proposed resistance design model was validated against the maximum bending moments of 74 RC and 69 PRC beams. Comparative analysis indicates that the predicted flexural capacities align well with experimental results. Furthermore, the validity of the combined method was verified using collected test datasets of RC/PRC beams, and partial safety factors corresponding to different failure modes were provided. The proposed method offers a practical approach for conducting effective reliability analysis of precast RC structures.</p>