The three-dimensional trap stiffness of optical tweezers formed with high-NA micromirrors is investigated by back-focal-plane interferometry and power spectrum analysis. Normalized stiffness values of kappa(xy)/P-trap =1.2(mu N/m)/mW and kappa(z)/P-trap = 0.52(mu N/m)/mW in the transverse and axial directions, respectively, have been measured for polystyrene spheres with a radius of 1.03 mu m. Compared with high-NA microscope objectives, micromirrors achieve much better trapping performances, particularly in the axial direction. (C) 2009 Optical Society of America