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The effect of crack depth (a) to specimen width (W) ratio, (a/W), and specimenthickness (B) on the initiation fracture toughness (JIC) and J-R curve of low carbon SA333Gr.6 steel has been investigated by three point bend test in the context of contradictory trendsreported in the literature. The fracture toughness tests have been carried out following ASTM– E1820, making provision for correct offset blunting line based on stretched zone width.The results indicate that JIC increases with increasing a/W ratio, but the increase is relativelysmall and becomes still smaller at lower thickness. The variation of the initiation fracturetoughness, JIC, with increasing specimen thickness shows that the JIC is more or lessinsensitive to increasing thickness, particularly at lower a/W ratio of 0.4 and 0.6 but at highera/W ratio 0.8, there is slight increase in JIC as the thickness increases from 15 mm to 25 mm.