Objective To investigate mechanism and clinical effect of implantation of pacemaker with artial fibrillation(Af) preventive pacing algorithms (Vitatron 900E).Method 24 cases of patients with paroxysmal Af were observed in three phases (each phase two weeks):⑴Monitoring phase,in which onset mechanism of atrial arrtythmias was recorded, the device was programmed into DDD mode with 60 bpm;⑵Lower rate limit phase (LRL),DDDR mode with 85bpm LRL, and evaluating anti-Af effect by elevated rate;⑶Preventive pacing therapy phase,all preventive pacing therapies with programming to on or off, and this phase determined whether combined pacing therapies impacted on incidence of Af. Results 24 cases of patients were free of Af, in which 17 cases were done by using post-PAC-response or/and post-exercise-rate-control therapy only, and 7 cases of 17 casees were done by combining it with cordarone 0 1～0 2g/d also. The other 7 cases were done by DDDR 85 bpm only.Conclusions Af prevention pacing may be benefit for patients with paroxysmal Af,especially with complicated symptomatic bradycardia.