Abstract: UHVDC (ultra\|high voltage direct current) transmission system generally has two 12\|pulse converters on each pole. In order to evaluate the reliability of HVDC transmission system, a kind of assessment approach to quantitatively calculate the reliability of UHVDC transmission system by using of state space method according to real application of UHVDC transmission system is proposed. The method takes topology and flexible operation schemes of a UHVDC system into consideration, which can reflect more accurate reliability assessment results and can provide practical reference for UHVDC system constructions and reliability management.