Micromachined membrane deformable mirror (MMDM) can serve as an “ad hoc” free-form optical element. To test the repeatability and stability of the standard MMDM, we have conducted the test of surface figure during multiple thermal cycling, test of figure drift at elevated temperatures, and a long-term 16-day stability test of actively formed mirror figure. The average rms error did not exceed λ/25 at λ/ 633 nm, after repeated cycling from -14 to +70° C, with return to the room temperature. The existing design provides λ/10 stability in the temperature range of ~ 10°. Optimization of the design, eliminating astigmatism, would allow one to extend the temperature range to about 30°. The long-term mirror figure instability at a constant temperature reaches λ/20 rms in 16 days. The P-V error with respect to the nearest sphere changes from λ/20 in the fiirst day, to about λ/10 in the 16-th day. The tests show that MMDM is stable enough to make a reasonable alternative to free-form optics in applications that require various optical shapes to be formed with a single element. © 2014 SPIE.