A new formula for computing ST reliability of radio-communication networks (RCN) from Source to Terminal t was presented. The new formula contains terms which correspond one by one to a class special ST path (SPRT). For a given networks, the terms of the new formula are fewer than those corresponding Inclusion-Exclusion principle. An algorithm for computing ST reliability was presented, it computed ST reliability or produced a ST reliability expression by enumerating a class of special networks of given networks. Because the structure of this class of new networks which need to be enumerated was relativity small, the new algorithm’s performance was better than Inclusion-Exclusion principle algorithm .Finally an example illustrates our conclusion.