Kinetic strength in grain growth is evaluated for HAZ of welded pipes. Austenite grain growth is dependant on the thermal cycle and the material properties. In this research the influence of thermal cycle is evaluated using characteristic time constant and characteristic temperature. These dimensionless parameters were calculated for the points lying in HAZ using temperature/time profile derived from Rosenthal equation for the heat transfer. The grain growth is assumed diffusion controlled and Arrhenius temperature dependency is applied for the rate of growth in grains. In an experimental investigation the welded pipes were sectioned longitudinally and the grain size in parent metal and HAZ were measured based on ASTM E 112 by using Planimetric Counting. The obtained data was compared with the result of the model and the appropriate kinetic constant which is calibrated the equation of grain growth is introduced for the seam weld and seamless pipes.