The objective of this study was to investigate the correlation between filler size, fracture toughness and voids. Seven model resin composites and one commercial have been used in the study. A single edge notch mould was used to prepare samples (n=8). A selected area of 1mm below and above the notch was scanned with micro CT and then the percentage of voids calculated. A universal testing machine was used to measure fracture toughness. Percentage of voids and fracture toughness data were analysed using ANOVA and post hoc methods were performed to check any significant differences between materials tested (p<0.05). Conclusion: Filler size is strongly correlated to % voids but has no effect on fracture toughness.