Based on the 1attice-structura1 hypothesis space of expert systems，we approach a computationa1 mode1 of evidentia1 reasoning under uncertainty: the Two-way Broadcasting Mode1 (TBM). In TBM，the measure of uncertainty of know1edge is represented as the certainty factor (CF) of the improved MYCIN model. This paper describes a efficient method which ca1cu1ates bpa functions from CF and prior probabilities. It a1so discusses the single node bpa's propagating mode1s，a extenting version of Shenoy-Shafer model.