In this paper, we evaluate performance of two types of piezoelectric diaphragm micro-actuators: open-end design and closed-end design. In the open-end design, the micro-actuator consists of a base silicon diaphragm, a layer of bottom electrode, a layer of lead-zirconate-titanate (PZT) thin film, and a layer of top electrode. The diaphragm is anchored on a silicon substrate by etching the silicon substrate from the back to form a cavity under the diaphragm. In the closed-end design, the bottom of the cavity is sealed with a piece of glass, silicon or PDMS. Experimental results show that the measured displacements from the closed-end design are always 5%-30% lower than those from the open-end design. To explain the experimental results, we hypothesize that the air inside the cavity of the closed-end design behaves like an elastic spring increasing the stiffness of the closed-end design. To confirm the hypothesis, we estimate the stiffness of the air by modeling the air as an ideal gas with a constant temperature. We also.