This paper presents a proof of the passivity of a two-degree of freedom flexible link manipulator. The proof of the passivity of the multi-degree of freedom flexible link manipulator is not yet done while many researchers tried to solve this problem. The difficulty of this problem is due to the complexity of the equations of motion of flexible link manipulators. The flexible manipulator is modeled as not a truncated finite dimensional model or approximated linear model but an infinite dimensional and nonlinear model. In our proof of the passivity, it is similar to the analysis of the rigid link manipulators. Homogenous matrices are defined to describe the motion of the manipulator then an inertia matrix of the flexible manipulator is introduced. And then the equation of motion of the flexible link manipulator is described in simplicity vector form by using the inertia matrix. A useful property such as a matrix appears in the vector form equation is skew-symmetric is obtained. By using this property, the passivity of the two-degree of freedom flexible link manipulator can be proved easily.