The performance of reinforced concrete structures is dependent on the bond between the concrete and the reinforcing steel. The loss of the bond strength decrease the strength of the reinforced concrete member with an increase service time of the structure. This study investigated the effects of loss of bond between concrete and rebar interfaces on the flexural behavior of slender reinforced concrete beam.The bond strength between reinforcement steel and concrete was determined using pull-out tests. Six 150mm ×150mm ×150mm cube specimen with embedded reinforcement in the center of the concrete cross-section were casted and examined. Three types of failures were observed which includes slippage failure, pullout failure, and splitting failure.Both experimental and finite element has been conducted to study the effect of different percentage of loss of bond flexural reinforced concrete beam. The bond strength between reinforcement steel and concrete for different loss of bond was determined in pull-out tests. The flexural behavior of six simply supported beams, each with 1000 mm total span, where one was full bonded beam used as control and five with the variable bond length between concrete and reinforcement interfaces, were tested under monotonic loading. The bond loss was introduced with plastic tubes surrounding the longitudinal tension reinforcement. The result of the study revealed that the bond loss between the reinforcement and concrete has significant effect on the mode of failure, crack pattern, ultimate load-carrying capacity and number of cracks on the flexural zone. All beams tested with loss of bond developed bottom surface cracks. The ultimate load-carrying capacity has been varied depending on the percentage of loss of bond. The crack patterns of beams analyzed using FEA were similar to experimental tests. Software simulated on beams shows small reduction in load but increment in deflection of reinforced concrete beam. Key words: Flexure, Reinforced concrete beam, Bond loss, Bond stress, Ultimate load, Deflection, Flexural strength, Crack pattern