Objective:To observe the clinical efficacy of CAD/CAM all-ceramic onlay in the restoration of molars with extensive defect after root canal therapy(RCT).Methods:260 molars with extensive defect were restored by all-ceramic onlay after RCT.The patients were followed up for 18 months.The restoration effects were assessed using USPHS criteria.Results:The success rate of CAD/CAM all-ceramic onlay was 98%.Restoration exfoliation was found in 3 cases,fractured in 2 cases.Conclusion:CAD/CAM all-ceramic onlay can be applied in restoring extensive postenor tooth-defection after RCT.