Abstract: Cognition and machining misunderstandings during the preparation process of round tensile test bar were analyzed,and correct understanding of transition radius r was emphasized. According to a number of national standards,appropriate sizes of the trial tensile bar were recommended and viable suggestions were put forward. The results show that when processing round tensile test bars,the transition radius size precision must be ensure and r≥ 0.75 d requirements of GB/T 228-2002 should be strictly implemented. If conditions permitting,larger transition radius r values should be selected first. D/d=1.50-2.00 department of clamp sizes were recommended and make D dimensions close to D=d+2r as far as possible.