The uniaxial true stress logarithmic strain curve for a thick section can be determined from the loadÐ diameter reduction record of a round tensile specimen[ The correction of the true stress for necking can be performed by using the well!known Bridgman equation[ For thin sections\ it is more practical to use specimens with rectangular cross!section[ However\ there is no established method to determine the complete true stressÐlogarithmic strain relation from a rectangular specimen[ In this paper\ an extensive three! dimensional numerical study has been carried out on the di}use necking behaviour of tensile specimens made of isotropic materials with rectangular cross!section\ and an approximate relation is established between the area reduction of the minimum cross!section and the measured thickness reduction[ It is found that the area reduction can be normalized by the uniaxial strain at maximum load which represents the material hardening and also the section aspect ratio[ Furthermore\ for the same material\ specimens with di}erent aspect ratio give exactly the same true average stressÐlogarithmic strain curve[ This _nding implies that Bridgman|s correction can still be used for necking correction of the true average stress obtained from rectangular specimens[ Based on this _nding\ a method for determining the true stressÐlogarithmic strain relation from the loadÐthickness reduction curve of specimens with rectangular cross!section is proposed[