Objective: The purpose of this study was to compare the retention of core build-up materials to a prefabricated dental post. Methods: Five post-core groups of 10 specimens each were examined. The core materials used in this study were Ti-Core, Ti-Core Natural, Ti-Core Auto E, Ketac-Silver GIC, and GC Miracle Mix GIC. The post used was Flexi-Post No. 2. For all test groups, the head of the post was placed into the core material at a 90-degree angle to the surface. The core material was allowed to set for 1 hour before being placed into distilled water for two weeks. Test specimens were then placed into a special jig and retention tests were performed using a universal testing machine. Results: The retention strengths were Ti-Core 926.6 ± 189.5 N, Ti-Core Natural 1034 ± 115.2 N, Ti-Core Auto E 879.8 ± 156.6 N, Ketac-Silver GIC 243.3 ± 55.6 N and GC Miracle Mix GIC 269.1 ± 58.3 N. One-Way ANOVA followed by the Student-Neuman-Keuls multiple comparisons test (p < 0.0001) revealed that the post head retention of the resin based core materials was significantly higher (4.2 to 3.3 times) than glass ionomer based core materials. The post head retention of the Ti-Core Auto E was statistically lower than Ti-Core Natural (85% of the Ti-Core Natural), but Ti-Core Auto E and Ti-Core were not significantly different from each other in terms of post head retention strength. Conclusions: In terms of post head retention strength, resin based core materials performed much better than glass ionomers. Though the Ti-Core Auto E had lower post head retention strength than Ti-Core Natural because of its higher resin content, its performance was still as good as Ti-Core.