Abstract: The true thickness of strata is conventionally calculated by using the scalar quantity in the field survey.In the paper it is proved that the true thickness of strata may be calculated by using M= L ·[sin β ·cos ε ·cos（ -λ ）+sin ε ·cos β ]if the true thickness of strata measured from the lower part upward along the traverse is regarded as the positive thickness while that measured form the upper part downward is regarded as the negative thickness.The ranges of values of the parameters in the formula are the same as those that may possibly appear in the field measurement of the section and all the data can be obtained directly from the records of the measured section.No data processing will be made and the calculation is simple and accurate.Through the calculation not only the true thickness of strata can be obtained but the relation between the upper and lower strata can be also reflected truely;so the thickness of strata has the nature of a vector.