Abstract: By using high strength steel TRIP 600 with multiphase microstructures and DC04 steel sheet for deep drawing as testing materials,the determination of plastic strain ratio by regression method presented in the ISO 10113 standard (2006) was introduced,with the discussion about the relationship between the regression method and conventional method,i.e. single strain pairs method as well. It was clearly shown that the evolvement tendency of r value was significantly influenced by the strain path,leading to the scatter of r value results by conventional method. The instability of yield deformation,the initial signal delay on the threshold of testing and the inertia effect when test speed switching were the major causes of strain error,which could be removed by the regression method. Considering the unsynchronization phenomena during longitude and transverse strain measurement,the regression line fitted in the selected range should not be set with the boundary condition,i.e. not through the original of coordinate.