The salt-bath annealing was used to study the recrystallization of structural steels as well as the effect of chemical compositions on the microstructure and properties of structural hot dip galvanized steel sheets was investigated in term with the production condition of the No.2 hot dip galvanizing line in Pangang. The optimum annealing temperature range was determined. Experimental results show that the recrystallization temperature of Grade40 and Grade33 is approximately 720°C and 680°C, respectively. Considering the stability of mechanical properties, their optimum annealing temperature are determined to be approximately 720°C. The recrystallization temperature of Grade50 is above 700°C. In order to ensure the margin of strength and the stability of properties of Grade50, the optimum annealing temperature is approximately 780°C.