A longitudinal force model of tire with two parameters was developed for estimating the longitudinal stiffness and rolling radius of the tire. The method of estimating two parameters was established by using a mixed technique of pattern search and least-square regression. The simulation program was made on Matlab platform with algorithm, and the validity of this algorithm was demonstrated by the perfect coincidence of the simulation and true results. Finally, the algorithm was applied to the vehicle real test data. It showed that measured tire pressure and estimated longitudinal stiffness are approximately inversely proportional. It also suggested that longitudinal stiffness estimate might be used to predict tire inflation pressure. 