The work included an experimental research of creep of cylindrical tensile test specimen, made of aluminum alloy D16T, at a constant temperature of 400ºС. The issue to be examined was necking at different values of initial tensile stresses. The use of a developed, non-contacting measuring system enabled to see dependence of changes to the specimen form and estimated true stress in various cross sections upon the time. Basing upon the experimental data obtained, criteria were proposed, to describe the point of time, when necking occurs (necking point). Calculations are given, at various values of parameters in these criteria. One has also determined a relative interval of deforming time, at which the test specimen is uniformly stretched.