Lead-free piezoelectric ceramics Na0.5Bi0.5TiO3+xmol%Co3+ (NBT-xC) were synthesized by conventional solid-state process. Effects of Co3+ addition on the phase structure, dielectric and piezo- electric properties of NBT-xC were investigated. The X-ray diffraction suggested that all specimens were rhombohedral perovskite structure. After a small addition of Co2O3, a dense microstructure with increased grains was developed. The doping with Co3+ enhanced mechanical quality factor Qm significantly, whereas piezoelectric constant d33 and electromechanical coupling factor kp decreased slightly with increasing amount of Co3+. For the ceramics with x=3, the electric properties become optimum, which are as follows: d33=~70pC/N, kp=~13%, Qm=~934, tanδ=~0.02 (1kHz), respectively. These results indicate that the Co3+ ion was considered to replace Ti4+ ion and present “hard doping” effect.