A simple one-step direct third-order nonlinear frequency conversion process in centrosymmetric crystals was studied, and an efficient third-harmonic generation in UV region was obtained. The expressions of phase-matching angles were determined. α-BBO crystal and the calcite crystal which both have the delocalized π bond were selected in our experiment. In type-Ⅱ phase-matched, the maximal output energy was 19.3 μJ and the maximal conversion efficiency was about 1.25% at 266 nm in calcite crystal, while the value was 37.6 μJ and 2.5% in α-BBO crystal. These results indicate the feasibility of direct third-harmonic generation in UV region, the possibility of direct third-harmonic generation in deep-UV region through centrosymmetric crystals, and also demonstrated a succinct and efficient way to generate deep-UV laser and a development direction of nonlinear crystals in UV region.