Abstract: Based on the principle of sizing,it is pointed out that the flat sizing roll bottom diameter progressive increment,Δd is a function of Dn —pipe O.D.,S —pipe W.T.,ΔD —sizing allowance and ε—tubular elasticity.Deduction of the flat sizing roll bottom diameter progressive increment is carried out on the basis of conversion of the sizing allowance into longitudinal increment of the pipe and of the fact that during sizing,the pipe has to be maintained with a specific stretch,and thus computing formula of the said progressive increment is established,which can be used as theoretic basis for proper design of the said progressive increment,meaning so much to prolonging the roll service span,improving pipe surface quality as well bettering the roll generality.