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This study presents a simple fracture model linking average grain size G of granite to tensile strength ft and fracture toughness KIC. This model only requires the peak load Pmax measurements of small notched samples to determine both ft and KIC. The influence of G was considered for: (i) quasi-stable crack growth before Pmax, and (ii) fracture transition from ft to KIC criterion. In-depth analysis was carried out on three-point-bending (3-p-b) results from granite with G ≈ 2.5 mm. The 3-p-b specimens have two different beam widths W = 30 and 70 mm, and the same span/width ratio S/W of 4. Another granite with G ≈ 10 mm reported in literature was also analysed to show the influence of different grain size on both ft and KIC. Comprehensive data on rock fracture are explained and compared with the results of this study to substantiate findings of this study. Experimental scatters in Pmax with different initial notch lengths a0 = 0 – 53 mm were analysed by normal distribution, following a recent study (Zhang et al., 2018). The new model with normal distribution predicted the mean and upper and lower limits with 96% reliability covering the experimental scatters.