Abstract: palaeoproductivitity is defined as the rate of energy absorbed by organism during the course of circulation of energy，that is，the quantity of organic matter per unit time and per unit area．The method of calculating palasoproductlvity in the ocean was studied by Müiller and Suess(1979)and the formula：R=C x×ρs×(1-φ)／(0．0030×s0.30)was obtained．This formulais deduced on the basis of surface sediments from Pacific Ocean，Atlantic Ocean, and Baltic Sea regions， and the majority of its sedimentary organic matter is of marine origin. Therefore，this for mula can only be used for sea regions where the nonmarine organic matters are basically absent．In our country most Mesozoic and Cenozoic basins containing oi1 and gas are nonmarine lakas．The organism transported to the lakes from the land occupies a large proportion．It has obvious difference from the organism deposited in ocean. Consequently，the relation(formula)between palaeoproductivity and organic carbon in the nonmarine 1akes should be rededuced．Based on the method of calculating palaeoproductivity in the ocean and using related sedimentologica l and biological data of modern nonmarine downfaulted basins in Yunnan Province，we obtained a formula for calculating palaeoproductivity using the method of organic carbon in the lake：R =C × ρs×(1-φ )／(0.00421 × S0.0826) Furthermore，we compared the palaeoproductivity values calculated by this formula with those calculated by the ocean formula for the Third Member of the Shahejie Formation of the Jiyang Depression．Shadong Province The results show that the productivity of the lakes is much higher than that of the ocean．This new formula is of significance in quantitative calculation of pa1aeoproductivity for the Cenozoic lakes，such as the Bohai Bay Basin which is a dovnafaulted basin．