This paper analyses the relation between longitudinal- and cross sectional-strain for plastic inhomogeneous deformation at the neck of tensile test specimen. The geometry of neck nrofile is described according to instantaneous measurements of diameters along the axis of the test specimen. An improved neck measuring instrument (MI) is proposed for ploting the neck profile during the test. Evaluating formulae corres-ponding to the proposed MI are deduced. The results of the proposed evaluating formulae for describing the mechanics of neck deformation are compared to those found elsewhere. The validity of the proposed method of evaluation is verified analytically and experimentally.