Damage parameter of concrete is widely used in design and construction of concrete structures. An experimentation method has been designed, which can simulate the initial damage by mixed initiator into concrete. Damage parameter can be obtained in laboratory with standard specimens. But practical structure is very large. It is not rational to use this parameter in practical structures. So size effect on damage parameter is studied. It is concluded that initial damage and summit damage grow with the increase of structure size, but the growth rate becomes gentle with size increasing. When the size is larger than 500 mm×500 mm, the growth rate is less than 15‰, so the results obtained from specimen in 500 mm×500 mm can be used for mass concrete structures.