ESR techniques have been used to study ZrO2 nanopowder (10-100 nm particle size) prepared by precipitation methods. They show that nano-ZrO2 has two kinds of ESR signals, which are assigned to Zr3+ and F-center paramagnetic centers, respectively. The Zr3+ Signal is a bulk related signal; on the contrary, the F-center signal is surface related. Quantitative ESR experiments indicated that the F-center signal intensity increases and the Zr3+ Signal intensity decreases with the increase of specific surface area : 50 nm is a critical particle size; when the nanoparticle size is bigger than 50 nm, its F-center signal vanishes, but its Zr3+ Signal intensity increases dramatically.