Many damage models of concrete have been put forward till now, of which some models are applied in engineering practices. The damage parameters play an important role in the calculations of damage with these models, which is determined during experiments. However, the tests of damage on concrete are rarely made, especially about the measuring method of initial damage. The initial damage is an essential parameter to study on the damage of concrete. Being based on the theory of damage, this paper analyze of the damaged and undamaged elastic modulus of concrete. The new method of determining undamaged elastic modulus from test results has been put forward, and then the initial damage has been quantified.