A new tensile test method to obtain a true stress - true strain curve was developed, in which a CCD camera and an image analysis device were equipped to enable the in-situ measuring of the change in shape of test specimen until the very end of the fracture. A smooth round bar tensile test specimen was adopted. The dynamic change in the necking geometry was observed continuously with a CCD camera and the measured data was simultaneously processed by an image analyzer. Tensile tests were carried out using SS400 steel and 5.0%Mn steel. The data of the radius of cross sections perpendicular to the specimen axis around the necking portion, the radius of curvature on the longitudinal section of necking surface, and the load applied to the specimen were obtained continuously until the specimen ruptures. The change in the minimum radius of the neck and the necking curvature were calculated from the data obtained, and they were confirmed to be accurate. According to the Bridgman’s model, true stress - true strain curve after necking can be obtained from the minimum neck radius, the neck curvature and the load. The data was analyzed based on the Bridgman’s model. True stress - true strain curves of SS400 and 5.0%Mn steels were determined by this method. True stress - true strain behavior after necking was thus elucidated. This newly developed test method is confirmed to be applicable for the accurate measurements of true stress - true strain curves until up to fracture of specimen.