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To verify the influence of fibreglass post diameter, as well as use of accessory posts on fracture resistance and bond strength, when used for restoring enlarged root canals.One hundred maxillary single-rooted canine human teeth were decoronated and root canals were prepared using a No.4 drill (White Post, FGM, Joinville, SC, Brazil). The roots were assigned to five groups: (EC1) post No.1 (Exacto Cônico, Angelus, Londrina, PR, Brazil); (EC2) post No.2 (Exacto Cônico, Angelus); (EC3) post No.3 (Exacto Cônico, Angelus); (ECA) post No.1 (Exacto Cônico, Angelus) plus two accessory posts (Reforpin, Angelus); (WP) post No.4 (White Post, FGM). Posts 1–4 have a crescent diameter. Posts were luted (Rely X-Arc, 3M ESPE, USA) and composite resin filling cores were prepared for the fracture resistance test (n = 10). For the push-out test (n = 10), roots were sectioned into 1 mm thick slices. Both tests were performed in a universal testing machine. Data were analysed using anova and Tukey's test.Groups WP and EC3 had higher fracture resistance than the other groups (P < 0.05), which were statistically similar. Root fractures occurred in 14% of the specimens. Groups EC1 and EC2 had lower bond strength values than Groups EC3 and ECA which were lower than the WP Group (P < 0.05). Bond strength was lower in the apical and middle third than in the cervical third of root canals (P < 0.05).Thicker posts were associated with higher resistance to fracture and bond strength to root canals. Using a post plus two accessory posts improved the bond strength but not the fracture resistance.