Based on the screw theory and the driven-chain principle,a methodology of structural synthesis for fully-decoupled two-translational(2T) and two-rotational(2R) parallel mechanism is proposed by analyzing the characteristics of the input-output relations for fully-decoupled parallel mechanisms.Firstly,according to the desired kinematic characteristics of fully-decoupled parallel mechanisms,a method is proposed by virtue of screw theory to synthesize the desired forms for both the direct and the inverse Jacobian matrices.Secondly,according to the feature of the direct and the inverse Jacobian matrices,the effective screws,the actuated screws and the mobile un-actuated screws of each leg are established based on the reciprocal screw theory and all possible topology structures fulfilling the requirements are obtained.Finally,the desired fully-decoupled parallel mechanisms can be synthesized by using the structural synthesis rule and structural synthesis of fullydecoupled 2T2R parallel mechanisms can be obtained exploiting the abovementioned methodology.In particular,the direct Jacobian matrix of each synthesized fully-decoupled 2T2R parallel mechanism is a non-zero diagonal matrix throughout the entire workspace.Motors are mounted on each leg and each one of them actuates one degree-of-freedom(Do F) of the fully-decoupled parallel mechanism through a one-to-one velocity relation. Based on the screw theory and the driven-chain principle, a methodology of structural synthesis for fully-decoupled two-translational (2T) and two-rotational (2R) parallel mechanism is proposed by analyzing the characteristics of the input-output relations for fully -decoupled parallel mechanisms. Firstly, according to the desired kinematic characteristics of fully-decoupled parallel mechanisms, a method is proposed by virtue of screw theory to synthesize the desired forms for both the direct and the inverse Jacobian matrices. Secondarily, according to the feature of the direct and the inverse Jacobian matrices, the effective screws, the actuated screws, the mobile un-actuated screws of each leg are established based on the reciprocal screw theory and all possible topology structures fulfilling the requirements are obtained. -decoupled parallel mechanisms can be synthesized by using the structural synthesis rule and structural synthesis of fully decoupled 2T2R parallel mech anisms can be obtained exploiting the abovementioned methodology. In particular, the direct Jacobian matrix of each synthesized fully-decoupled 2T2R parallel mechanisms is a non-zero diagonal matrix throughout the entire workspace. Motors are mounted on each leg and each one of them actuates one degree-of-freedom (Do F) of the fully-decoupled parallel mechanism through a one-to-one velocity relation.