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In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instabihtv strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.