The fracture toughness of rock often varies depending on the specimen shape and size. We used the Semi-Circular Bend (SCB) specimen to investigate the effect of specimen size on mode I fracture toughness. For comparison, the fracture toughness of the Short Rod (SR) specimen was also investigated, which is one of the testing methods suggested by the International Society for Rock Mechanics (ISRM). Two types of sandstones with different tensile strengths were used. The test results indicate that the fracture toughness tends to increase with the specimen radius. The value of fracture toughness for the SCB specimen is almost the same as that for the SR specimen when the SCB specimen radius satisfies the minimum size requirement. On the other hand, fracture toughness may be underestimated when the SCB specimen of smaller size is used. The results indicate the existence of the minimum size limitation of the SCB specimen.