Abstract: Here in the article is dealt briefly With the conception 'converted cost, and economical pipe size'.An ecoominc mathmatic model is established and programmed in BASIC language based on gas flow rate, pipeline strength, compressor power and economic calculation formulars, analying the effect of various varieties upon economical pipe size. It is concluded dy pointingi tuothe dia lectical relationship between the three factors of gas output, pressure and price, and the economical pipesize.