In this paper a process of determination for thyristors operating with optimal angle is described. The proposed method is based on the trade-off between the cost(due to rating and loss) and controllability of the system. Large control angle increases controllability of HVDC system and also increase the rating of the system which leads to a high price. Therefore it is important to determine a control angle at a first step of design procedure. This paper shows a rating trend of C.Tr, Valve and Filter due to control angle variation, and then introduces a determination process of the HVDC operating angle.