Boundary effect and size effect of fracture toughness and strength of mortar were studied by three point bending test and strength test for mortar specimens with different sizes. Results show that the strength of specimens decreases with increasing block size. If the virtual crack formation at peak load is not considered, the linear elastic fracture mechanics(LEFM) condition can be satisfied until the height of the specimen reaches 047m. On the contrary, if the virtual crack formation at peak load is considered, the specimen height increases over 050m, it is closer to but not large enough to fully ensure the LEFM condition. The concept “relative size” is proposed to study the size effect, which is distinctive compared to those existing size effect model only connected to specimen size. The effect of aggregate size of mortar on quasi brittle fracture is considered and an associated method for determining simultaneously the fracture toughness and bending strength of mortar is proposed, in which both the fracture toughness and bending strength satisfy the LEFM condition and have no size effect.