Lead-free ceramics K0.5Na0.5NbO3-0.5 mol%Al2O3 with simple composition were prepared via the hot-press sintering. The ceramics exhibit dense microstructure, fine grains, and pure orthorhombic structure. Via the hot-press sintering as well as adding 0.5 mol%Al2O3 into K0.5Na0.5NbO3, piezoelectric and ferroelectric properties of the obtained ceramics were obviously enhanced. The ceramics exhibit high remnant polarization P-r = 26.8 mu C/cm(2) and piezoelectric constant d(33) = 137 pC/N. The maximum unipolar strain is 0.17%, and the converse piezoelectric coefficient d(33)* is 212 pm/V under electric field E = 80 kV/cm. Under E = 50 kV/cm, the P-r increases from 21.6 to 22.2 mu C/cm(2), and the d(33)* increases from 223 to 322 pm/V as the temperature increases from room temperature to 170 degrees C. The ex situ measurement result demonstrates that d(33) still keeps a high value of 97 pC/N as the annealing temperature increases to 380 degrees C, implying excellent thermal stability.