On the basis of a summary of achievements from a large quantity of experimental studies both at home and abroad, in combination with the authors'experience in the experimental works on the reinforced concrete frame joints, their force-transferring characteristics are fully studied. The important fact that there is also a confined mechanism besides the inclined compressive strut and the truss mechanisms,is clearly pointed out. The influences of the three mechanisms upon the mechanical behaviors of the joint core concrete, the transverse and the vertical reinforcements,as well as upon the failure characteristics of the joint are also explored. Mechanical behaviors of the inclined concrete compressive strut and the factors causing its failure are specially discussed. These analyses provide theoretical bases for establishing more rational earthquake-resistant design criterion for reinforced concrete frame joints.