An experimental research on the influence of defects in recycled aggregate on mechanical properties of recycled aggregate concrete（RAC） was conducted. Concretes with two waterbinder ratios（0.59 and 0.25）were broken into recycled aggregate（RA）， which might induce defects in the forms of damage of gravel in RA and mortar attached to RA. A type of thermal treatment， i.e. heating under temperature 620℃ for 3 h， was employed to remove attached mortar. Tests were conducted to determine compressive strength， tensile splitting strength and fracture energy of the concrete. The experimental results reveal that the mechanical properties of RAC are significantly lower than those of natural aggregate concrete. As to RAC with a low waterbinder ratio， gravel damage is a main factor causing the decrease in the mechanical properties， while with a high waterbinder ratio， attached mortar is such a factor. Among the properties measured in this investigation， water absorption and fracture energy are the two factors that can sharply reflect influence of the defects in RA during the broken process. It is suggested that prior to mix proportion design and preparation of concrete using recycled aggregate， it is necessary to obtain the details of demolished concrete such as its strength grade or waterbinder ratio， and of the characteristics of defects in recycled aggregate.