The optical transmittance of pressureless sintered KZr2P3O12 (KZP) ceramics was studied in the ultraviolet, visible light, and near infrared spectrum ranges. Thermal expansion, strength, a nd therm a I shock resistance of the ceramics were also studied. KZP ceramics with different additives (0-8 wt % of MgO, ZnO or SrO) show a 30-70% transmittance from 0.4-2 mu m, a low thermal expansion coefficient and high thermal shock resistance. These polyfunctional ceramics might be useful as optical windows.