Aurivillius ferroelectrics K_0.25Na_0.25La_0.5Bi₂Nb₂O₉ (KNBN-La) and K_0.25Na_0.25Bi_2.5Nb₂O₉ (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 (T_c) of about 360 °C. The KNBN-La ceramics had a high remnant polarization (P_r) of 13.6 μC cm⁻² and a field-induced strain of up to 0.031%. Particularly, the decrease in P_r for the KNBN-La ceramics after 10⁸ cumulative switching cycles was only 6%.