Abstract: Serial impact test datas were fitted with Boltzmann function and hyperbolic tangent function mathematical model separately and the ductlle-brittle transition temperature curve were gained. Quality and applicability of these two models were compared and parameters selection of these two models were discussed. The results show that mathematical model of Boltzmann function and hyperbolic tangent function were two different expressions of the same function. They had equal effects on ductile-brittle transition curve fitting. When high level value and low level value were not obvious,it was the precondition of determining accurately ductile-brittle transition temperature to suppose logically high level value and low level value.