Abstract: The rod-pinch diode is a cylindrical pinched beam diode that generates an intense pulsed small diameter bremsstrahlung source for flash radiography. If the pulse power source has the inductive voltage adder structure in vacuum vessels, the mechanical deformation caused by its long cantilever arm and air pressure could induce visible concentricity deviation to the anode and cathode structure. To evaluate that the concentricity of the cathode and anode geometrical centers does have some effect on the physical characteristics of the rod-pinch diode and the gap closure, a positive polarity rod-pinch diode was designed to work at 1 MV, and three different values of eccentricity were used in experiment: less than 1%, 15.06%, 22.92%. The experimental results show that the diode impedance decay rate is increased with a larger concentricity deviation. This implies that the electrode plasma expansion is fast. A larger concentricity deviation results in the impedance mismatch and decrease in energy coupling efficiency between the diode and pulse power source at the magnetically limited phase.