Abstract: The calculation formula about the true thickness of thestrata proposed by L mu of the U. S. S. K. is m=L. （sina·cosβ·sinγ+cosα·sinβ）. The author changes the included angler of the measuring direction and the str ike of the strata in the formula into the included angle ω of the measuring angle and the dip of the strata and changes the "±" method in the formula into the unified "±" method. The improved formula is as follows: m=L·（sinα· cosβ·cosω+cosα·sinβ） The improved formula has two characters: （1） it is applicableto the common function and programme-controlled calculator or computer as an operation tool and continuous operation, thus keeping the precision from being affected by the rounding off of the decimal place and consultation of tables and improving the precision of calculation; （2） the measuring data are consistent with the input data, thus attaining the aim of simplifying calculation.