A microdeformable mirror was designed. One important design issue was whether or not the gap between substrates would be airtight. Another design issue was whether residual stress and annealing would affect the deformation. A theoretical description of the deformation of the mirror was obtained by means of the ideal gas law and the theory of plates and shells. It was found that an airtight gap would require application of greater voltages to deform the mirror, but would help the device avoid pull-in phenomena. The results showed that an annealed mirror would have a larger deformation and would be reliably useful even with a large voltage. To test the theory, an electrostatically actuated microdeformable focusing mirror was fabricated by bulk micromachining. To avoid high-temperature bonding, SU-8 photoresist was used as a wafer bonding layer. The results show that electrostatic force can be used to deform the mirror into a parabolic shape and that the parabolic mirror focuses light well. The theoretical predictions were confirmed by the experimental results.