To help with the planning and design of distribution automation systems（DASs），the best amount of terminal units（TUs）mounted in a DAS is analyzed from the perspective of benefit/cost ratio first.From the perspective of reliability，the amount of various kinds of TUs in different situations is studied，that is only TUs with tele-control function，only TUs without tele-control function，a hybrid of TUs with tele-control function and TUs without tele-control function.Some circuit breakers with current protection mounted on some branches are investigated at length.The following conclusions are drawn that only one TU for each feeder is the best choice from the viewpoint of benefit/cost ratio，and that the demanded amount of various kinds of TUs depends on the reliability performance desired，the time of isolating the fault section by manual operation，the time of reparation，and the frequency of failure per-year.A DAS planning example with220feeders is given to illustrate the proposed methodology.