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• Several design guidelines for FRP flexural strengthened RC beams have been compared. • The model error of each design guideline was evaluated based on a database. • The reliability index was obtained by the importance sampling (IS) method. • The calibration of design factors was performed with different target reliability indexes. In the past few decades, many guidelines and standards for FRP strengthened reinforced concrete (RC) structures have been presented based on the theory of reliability-based limit state with the load and resistance factor design (LRFD) approach. The LRFD method consists of two formats: the resistance reduction factor format and the material partial safety factor format. In this paper, several well-recognized design guidelines with these two formats for FRP flexural strengthened RC beams have been assessed based on model error and reliability index. First, the model error of each design guideline was evaluated based on a collected test database. Then, the reliability index was obtained by the importance sampling (IS) method, in which the optimum density function is determined by the first-order reliability method (FORM). Finally, the calibration of resistance reduction factor or FRP partial safety factor was performed with different target reliability indexes. The results show that: (1) The model error of each failure mode is different for each design guideline. For flexural failure, China's GB 50608 2010 has the lowest model error. For intermediate crack-induced (IC) debonding, UK's TR 55 2012 shows the highest model error. For end debonding, Italy's CNR-DT 200 R1/2012 is less conservative. (2) It is difficult to meet the pre-given reliability level for some failure modes in several design guidelines using the originally suggested design parameters. For example, GB 50608 2010, fib T5.1 2019 and JSCE 2001. (3) The calibrated resistance reduction factors or the FRP partial safety factors are provided to satisfy the target reliability level. However, it is meaningless and uneconomical to adopt partial safety factors that are too large or reduction factors that are too small to fit into a design. Other options can be chosen such as applying additional anchorages to avoid end debonding, increasing the FRP bonding area, and reducing the FRP thickness to avoid the probability of IC debonding failure. [ABSTRACT FROM AUTHOR]