Abstract: This paper discusses the conditions of translation from a thermally wall body into a thermally thin body, the former the temperature is both functions of time and space, the later the temperature is single function of time under connective heat tranfer circumstance. On the bases of analysing the reasonableness and limitation of all criteria of thin wall body at present, a new criterion of thin wall body has been developted, in which the largest dimensionless temperature gradient is considered less than a certain constant, and the effect of Fo and Bi numbers on the new criterion has also been demonstrated.