Abstract: Orthogonalization method of simultaneous estimation of polynomial model order and parameter is presented in this paper. The coefficient matrix is orthogonalized by using polynomial and the overdetermined equations are solved by Householder transformation. The numerical ill-condition is decreased as low as possible, hence the operational accuracy is increased. The least square cost functions of several different model order can. be obtained in the process of parameter estimation by means of specially arranging the information matrix and hence the model order and parameter can be simutaneously estimated.