Purpose: The purpose of this study was to test the hypothesis that nucleation temperature significantly affects the fracture toughness of a fluorcanasite-based glass-ceramic. Materials and Methods: Sixty specimens were cut from a glass bar, polished, and randomly divided into six groups for nucleation treatment at temperatures of (1)520°C, (2)550°C, (3)580°C, (4)610°C, (5)640°C, and (6)670°C for 4 hours and a crystallization temperature of 850°C for 6 hours. A precrack was produced at the center of each bar, and the prepared specimens were subjected to three-point flexural loading with the cracked surface under tension using an Instron machine at a cross-head speed of 0.5 mm/min. Fracture toughness was calculated based on the indentation strength technique, and crystal volume fraction was determined by quantitative stereology of SEM images of each group of ceramic specimens. Results: The mean fracture toughness and crystal volume fraction ranged from 2.6 to 3.5 MPa·m1/2 and from 65% to 81%, respectively, within the limits of the nucleation temperatures investigated. ANOVA showed statistically significant differences among the test groups. Based on Duncan’s multiple comparison test, significant differences in mean fracture toughness and crystal volume fraction were found among the following statistical subsets: groups 1 to 4, group 5, and group 6. Conclusion: Fracture toughness and crystal volume fraction of a fluorcanasite-based glass-ceramic were strongly influenced by nucleation temperature; the crystals precipitated during thermal processing are thought to be an important factor in increasing fracture toughness. Int J Prosthodont 2003;16:505–509.