The major part of existing bridges in our country is concrete bridges, having serviced for about 30 years. Damages, deteriorations and even disaters due to design, construction and long time service, such as cracking, corrosion, and load capacity deficiency, could be found easily on the existing bridges, which may heavily influence functionality and safety of the structures in road systems. So it is necessary using state-of-art techniques to inspect and assess in-service bridges efficiently. Furthermore, the development of bridge management systems in our country is lagged comparing with developped countries. We have little mamangement experience and few historical data of existing bridges, and lack efficient assessment methods for structures. And the structural assessment results are always significantly influenced by assessors' empirism and subjectiveness. Therefore, this dissersation was mainly focused on the study of condition state assessment system for RC beam bridges, which includes the followings:The framework of condition state assessment system for RC beam bridges (CBCSAS) was estabilished based on the combination of the fuzzy set theory and artificial neural networks. The CBCSAS is a modular system, including database, inspection, structural assessment, main cause identification and priority-to-action modules.Due to the fact that the current bridge condition state rating system is highly subjective in practical application, this dissertation tailored a specific rating system for bridge condition states based on the national codes, considering the basic features of structural condition state rating. Comparing with the current Techincal Specification of Highway Bridges, the assessment system estabilished in this study is fully based on the results of systematic calculation, which significantly minimize the subjective disturbulence in the process of condition state assessment of concrete beam bridges.Based on the principle of measurable, representive and sensivity a hierarchical assessment criteron system, including "bridge age" and "environment" which were unable to be considered in the assessment method recommended by the the current Techincal Specification of Highway Bridges, was set up for RC beam bridges after fully study of several damaged bridges. For each criteron, then, the reference description the knowledge expression in CBCSAS