This study is aimed to investigate the effects of the specimen size and prefabricated crack length on the rock fracture toughness. Then three-point bending tests are conducted on semi-circular limestone specimens（NSCB） with four disc radii of 25.0

37.5

50.0 and 75.0 mm

respectively. The ratio of prefabricated crack length to the radius of disc specimens（dimensionless crack length） ranged from 0.3 to 0.7. The results show that the failure process of NSCB specimens exhibits the characteristic of an obvious brittle failure. Compared with the prefabricated crack length

the specimen size has more significant influence on the peak load and the shape of load-deformation curve. As the specimen size increases

the testing values of fracture toughness increase continuously.Moreover

the increase rate increases with the length of the prefabricated crack length. For a selected-size specimen

the testing value of fracture toughness reduces with the increase of prefabricated crack length. In order to reduce the effect of size effect in the fracture toughness test

it is suggested that the radius of NSCB specimen is greater than 37.5 mm and the dimensionless prefabricated crack length is in the range of 0.4～0.6. This study provides certain references for the promotion of the international method and the accurate test of rock fracture toughness using NSCB specimens.