Sintering behaviors of (Pb, La)(Zr, Ti)O3 (PLZT 8/65/35) ceramics in ozone atmosphere and their ferroelectric properties were investigated. In the early stage of sintering, the presence of ozone prompted the growth of crystallites and grains, and promoted densification. Ozone content dependence of these structural factors of ceramics influenced their optical transmittances, and ferroelectric and electrooptic properties. Marked improvement of electrooptic coefficients (R = 30 × 10-16 m2/V2, Vπ = 10 V with 0.40-mm-thick plate) was obtained for ceramics sintered at 1100°C for 24 h in an ozone-rich (180 ppm) atmosphere, as compared with ozone-free ceramics.