AbstractThe adhered mortar attached on the surface of recycled aggregate is the primary difference between recycled aggregate and natural aggregate, and is the major reason for reducing its mechanical properties. To measure the percentage of adhered mortar, an image analysis method (IAM) was presented. First, three comparative specimens were conducted to demonstrate the precision and efficiency of IAM. Further, the actual adhered mortar content was also studied by the heating and cooling method. Then, the effect of recycled aggregate size on the percentage of adhered mortar, and the characteristic of recycled aggregate was explored by IAM. In addition, the type and optimal size of recycled aggregate were also investigated. The obtained data indicated that IAM is an accurate and efficient method to measure the proportion of adhered mortar or other phases; the highest content of adhered mortar was found in recycled aggregate with a size of 10.0–16.0 mm, up to 39.39%; the percentage of the recycled aggregates with a particle size between 16.0 and 20.0 mm should be increased in the mix proportion, which has less adhered mortar content and higher quality recycled aggregate content.