Aurivillius ferroelectrics K0.25Na0.25La0.5Bi2Nb2O9 (KNBN-La) and K0.25Na0.25Bi2.5Nb2O9 (KNBN-Bi) were prepared by using solid-state reaction process. Rietveld refinements for the KNBN-La and KNBN-Bi were carried out by using powder X-ray diffraction at room temperature and they were confirmed to be two-layer Aurivillius oxides with orthorhombic space group A21am. The lattice parameters are a = 5.50468(10) Å, b = 5.49217(10) Å, and c = 25.05108(35) Å for KNBN-La and a = 5.48867(6) Å, b = 5.47895(6) Å, and c = 25.10591(25) Å for KNBN-Bi. Lanthanum (La3+) substitution for bismuth (Bi3+) led to an enhancement in relaxation behavior for the KNBN-La ceramics, with a ferroelectric to paraelectric phase transition temperature (Tc) of about 360 °C. The KNBN-La ceramics had a high remnant polarization (Pr) of 13.6 μC cm−2 and a field-induced strain of up to 0.031%. Particularly, the decrease in Pr for the KNBN-La ceramics after 108 cumulative switching cycles was only 6%.