The theoretical and experimental study on the single-longitudinal-mode of an injection-seeded optical parametric oscillator is presented. The numerical result shows that the frequency of the parametric output was determined by the longitudinal mode frequency of the cavity, if the frequency shift of the seeder was smaller than the axial mode separation. It was supported by an experiment on a single-longitudinal-mode BBO OPO which was seeded by a multi-longitudinal-mode He-Ne laser.