ABSTRACT This paper presents a kinematic and dynamic modeling of a three-port transmission mechanism using the block diagram technique. By exploiting the simplicity of the block diagram technique, unlike the more complex conventional approaches, the three-port transmission mechanism is systematically characterized to obtain the velocity kinematics and torque relationship. This is achieved by establishing a feedback connection of the three-port transmission mechanism in block diagram form, and obtain the close-loop input/output relationship using techniques known in the control community, such as Mason’s rule. Two categories of the three-port transmission mechanism were investigated including, the 1D rotation type (planetary gear) and the 2D rotation type (differential). The kinematic and dynamic expressions of both categories were shown to be easily obtained, identifying the characteristic differences. The velocity kinematics, torque relationships, and power analysis were expressed to validate the proposed modeling. Further verification using MATLAB Simulink shows a good agreement of the analytical expressions and the proposed block diagram model. To demonstrate the effective use of the proposed approach, the block diagram technique is applied in the design analysis of a south-pointing chariot device.