Light rare earth metal (LREM,La,Ce)films of 1500? thick were deposited on Si(lOO) substrate at room temperature under UHV condition. And a variety of techniques including AES,XRD and SIMS were used to study the thin film contact reaction between LREM and Si after isothermal annealing in the range of 200-400℃ for different durations. A new thin film contact reaction mechanism was observed. Taking into account the microscopic interfacial reaction, a model was suggested to elucidate this new reaction mechanism.