Ultrasonic sensors fabricated using (K,Na)NbO₃–KTiNbO₅ (KNN–NTK)-based lead-free piezoelectric ceramics were fabricated, their sensor characteristics were compared with those of sensors fabricated using Pb(Zr,Ti)O₃ (PZT)-based piezoelectric ceramics, and effects of material properties on sensor performance were investigated. The sound pressure, sensitivity, temperature increase during driving, and the responsiveness of the KNN–NTK-based sensor equaled those of the PZT-5A-based sensor. PZT-5A is generally used for ultrasonic sensors and has the same mechanical quality factor Q_m as the KNN–NTK ceramics. Most sensor characteristics are explained based on the electromechanical coupling factor and Q_m. However, the change in frequency characteristics under high applied power was observed that is attributed to the tendency of the nonlinear phenomenon of KNN–NTK-based ceramics to be weaker than that of PZT-based ceramics.