Lead-free piezoelectric ceramics (KxNa1?x)1-yLiyNb0.80Ta0.20O3 ( x=0.40–0.60 when y=0.030, 0.035, 0.040 ) were prepared by conventional solid-state reaction method to investigate the thermal stability of piezoelectric properties. It has been found that samples in the researched compositional range have high piezoelectric constant of d33=250 pC/N and kp=50%, respectively. In the thermal aging test up to the Curie temperature of approximately 325oC, d33 and kp of the ceramics remain almost unchanged although there existing the polymorphic phase transition (PPT) near room temperature. Moreover, ceramics presents the very good piezoelectric temperature stability and kp show the weak temperature dependence over a wide measured temperature range from -50oC to 120oC. The excellent thermal stability was discussed from the aspect of temperature range of phase coexistance.