In order to investigate the effect of specimen size on the mode I fracture toughness using Semi-Circular Bend (SCB) specimen, we experimentally studied the fracture toughness using SCB and Short Rod (SR) specimens, both of which are International Society for Rock Mechanics (ISRM) suggested methods, for comparison. Fracture toughness of a strong sandstone having an unconfined compressive strength of 145 MPa was investigated. The test results revealed that the fracture toughness increased with specimen diameter and reached a limiting value. These experimental results further showed that the value of fracture toughness determined using SCB specimen is almost exactly the same as that using SR specimen, when the SCB specimen satisfies the minimum size requirement. However, SCB specimen being one without a chevron notch unlike SR specimen gives lower fracture toughness if the minimum size requirement is not satisfied.