The statistical data about the operation reliability indices of the national HVDC transmission projects during the years of 2001 to 2005 and the historical data abroad from 1998 to 2002 are summarized. Through analyzing and comparing among reliability indices of these HVDC projects， the main factors influencing reliabilities of systems and the tendencies of these factors are clarified. Using the resulting information including HVDC projects abroad， the paper then makes a comparison among reliability indices of HVDC projects with two converters per pole and single converter per pole， respectively. The statistical analysis and conclusions are capable of being used as reference to decision making for the operating， planning and management of HVDC projects.