High strength low alloy steel in direct quenched and tempered condition was welded using gas shielded arc welding process at different welding heat inputs. The austenite grain size adjacent to fusion line was measured in order to calculate the apparent activation energy Q_app for grain boundary movement, the time exponent n and the kinetic constant K. The austenite grain size in heat affected zone was evaluated using these calculated parameters, and was compared with the austenite grain size measured in real heat affected zone of joint. The microstructure changes in heat affected zone were studied. Results indicated that the calculated parameters can be used to the predication of austenite grain size in heat affected zone area heated to a temperature of above 1100°C. The prediction results have a good agreement with measurement, while calculation at low temperatures was poor.