The study of loss of the tensile stress of the steel bar by rust corrosion was proceeded by select the sample steel bars which be corroded to analyze the physical character and find the percentage of weight decrease. Then made the comparison with the steel bar – weight of Thai Industrial Standard 20-2543 (RB) and 24-2536 (DB) and tested the resistance of tensile force. The result found that the steel bar that had percentage of weight decrease at 1-2% would had percentage of tensile stress decreased at 9-15% and had percentage of stretch at 9-39%. When they were compared with the Thai Industrial Standard 20-2543 (RB) and 24-2536 (DB). Overall if the percentage of the weight of the steel bar was decreased at 1-2% the steel bar would can be used in building with unimportant structure such as ground slab by rub the rust off and had the sampling random for the strictly standard test