Objective ·To explore the appropriate strategy for restoring maxillary first molars with palatal-occlusal (PO) defect or distal-occlusal (DO) defect by using fiber-reinforced composite posts. Methods ·Two types of defects in maxillary first molars were established: PO defect and DO defect. For each type, 5 finite element models with different restoration strategies were created: no post (NP), palatal post (PP), palatal and distobuccal posts (PDP), palatal and mesiobuccal posts (PMP), and palatal, distobuccal and mesiobuccal posts (PDMP). In the multi-post groups, if 2 posts overlapped in the resin core, the thinner one was horizontally trimmed 1 mm below the intersection point. The models were loaded by a vertical force—an 800 N force parallel to the long axis of the tooth, and a lateral force—a 225 N force directed at 45° to the long axis of the tooth. The following parameters were calculated by using finite element analysis: equivalent stress in the tooth structure and the posts, and maximum shear stress on the post-cement and cement-canal interfaces. Results ·Under the vertical loading, the maximal equivalent stress on the external surfaces of the tooth with PO defect was the lowest in the PMP group (36.17 MPa), while it was the lowest in the PDP group with DO defect (36.23 MPa). Under the lateral loading, it was the lowest in the PDMP group with PO defect (40.47 MPa), while it was the lowest in the PMP group with DO defect (42.05 MPa). With either defect, the equivalent stress on the internal surfaces generally decreased at the cervical 1/3 of root canals and increased at the middle 1/3 after post inserting. Palatal canal post and mesiobuccal canal post respectively withstood the highest equivalent stress under vertical loading and lateral loading (60.75?71.29 MPa and 45.91?51.82 MPa, respectively), and the maximal shear stresses on these two post-cement interfaces were also the highest under the corresponding loading (11.26?12.93 MPa and 12.38?13.03 MPa, respectively). The maximum shear stresses on the cement-canal interfaces were similar among the groups under the vertical loading (9.96?10.58 MPa), while under the lateral loading they were higher in the PMP group and the PDMP group. Conclusion ·The appropriate strategy for fiber-reinforced composite post restoration on maxillary first molars should be determined according to the type of tooth defect. For PO defect, the strategy of one post restoring in palatal canal is recommended; for DO defect, the strategy of two posts restoring in palatal and mesiobuccal canals respectively with approaches to reduce vertical occlusal force is recommended.