Objectives:Fibre posts are frequently used for esthetic restorations. Theaim of this study was to evaluate the effect of cyclic loading on the bondstrength of the fibre post D.T. Light Post using different post diameters. Methods: Sixty caries free single rooted teeth that were selected forstandardized size and quality were endodontically prepared and coronallyreduced to the cemento-enamel junction. Post holes were prepared using thecorresponding burs (10 mm in length). The specimens were randomly assignedto three experimental groups: (A): insertion of DT Light Post #1, (B): DTLight Post #2; (C) DT Light Post #3. Fibre posts were luted with Panavia Faccording to the manufacturers instructions. The restored teeth were storedin water at 37°C for at least 1d. Tensile tests were performed using auniversal testing machine. Each group was subdivided into two subgroups (1and 2): 1: immediate loading to maximum bond strength; 2: maximum load after100 cycles (between 10 and 20 N). Data were analyzed with SPSS 12.0. Results: Following mean retentive strengths were evaluated: Analysis ofvariances test revealed that the post diameter did not affect the bondstrengths the fibre posts of the D.T. Light Post system (pconclusions: thed.t. light post system showed acceptable tensile bond strengths. cyclicloading decreased the bond strengths of all diameters of fibre postsused. Keywords: bond strength, fibre posts, cyclic loading