To improve the temperature stability and electrical properties of KNN-based ceramics, we simultaneously consider the phase boundary and the addition of rare earth element (La), 0.96K(0.5)Na(0.5)Nb(0.96)Sb(0.04)O(3)-0.04(Bi1-xLax)(0.5)Na0.5ZrO3 (0x1.0) ceramics. More specifically, we investigate how the phase boundary and the addition of La3+ affect the phase structure, electrical properties, and temperature stability of the ceramic. We show that increasing the La3+ content leads to a change in phase structure, from a rhombohedral-tetragonal (R-T) phase coexistence to a cubic phase. More importantly, we show that the appropriate addition of La3+ (x=0.2) can simultaneously improve the unipolar strain (from 0.127% to 0.147%) and the temperature stability (i.e., the unipolar strain of 0.147% remains unchanged when T is increased from 25 to 80 degrees C). In addition, we find that the ceramics with x=0.2 exhibit a large piezoelectric constant (d(33)) of similar to 430 pC/N, a high Curie temperature (T-C) of similar to 240 degrees C and a fatigue-free behavior (after 10(6) electric cycles). The enhanced electrical properties mostly originate from the easy domain switching, whereas the improved temperature stability can be attributed to the R-T phase boundary and the appropriate addition of La3+.