Abstract: Weymouth formula has been applied to technological design of sichuan gas trunk pipelines. It has been proved by practice that the value obtained by using the formula is far differnt from that measured, with an error being around 30-40 %, sometimes eves higher than 50 %. In consideration of this, the author chooses the basic parameters and the data measured about such three gas trunk pipelines as Yong Xing-De Yang Xin, Cro Jia Ba-Yue Xi, and Liu Bin-Deng Guan in Sichuan, and derives the experimental formula for hydraulic friction coefficient in hydraulic smooth area as well as mixed friction area from solution obtained by diagrams, with revision made of the Weymouth formula for square friction areas. The three formulas mentioned above are put in the data actually measured about the three pipelined, and the resultg show that the value obtained in the experimental formula is much closer to the value measured in comparison with Weymouth formula. It is obvious that the experimental formulas can reflect better the actural situation of Sichuan gas trunk pipelines.