In recent years, the deterioration of concrete structures has become a public problem and maintenance and management techniques have been subjected to intensive research. For maintenance it is important to evaluate not only strength of concrete but also other characteristics such as degree of damage. Using acoustic emission (AE) and damage mechanics can make quantitative damage assessment of concrete. In the present study, early-age concrete, long-term cure concrete, and concrete damaged by freezing and thawing process are examined in order to assess a quantitative damage evaluation method. It is found AE behaviors of concrete differ depending on the degree of damage, and could be simulated by using the rate processanalysis. Introducing a Loland's model, a relationship between stress and strain is approximated, and the suitability of the damage parameter Ω is confirmed. By calculating the relative damage, the initial damage Ω0 of an actual structure is successfully estimated.