Lead–zirconate–titanate PbZr(1−x)TixO3 (PZT), materials for piezoelectric applications are the dominant piezoceramics since their properties are high. However PZTs need to be substituted by safer materials due to health and environmental problems of lead. In this work, the crystal structure modifications of BaTiO3 induced by cobalt doping and niobium/cobalt co-doping were investigated in the range 0.1–25 at%. Low Co and Co/Nb doping level significantly improve piezoelectric properties of BaTiO3. The piezoelectric constant, obtained for BaTiO3 doped with 0.075 at% of Cobalt (d33=200 pC/N), is twice of that measured for pure BaTiO3. At high doping concentration, the structure of BaTi1−x CoxO3 and BaTi1−x CoxNbxO3 change from tetragonal to hexagonal while BaTi1−x CoxNb2xO3 remains tetragonal and gradually turns into cubic. It suggests that the ionic charge defect is balanced by oxygen vacancies formation when Ti4+ are substituted by Co2+/Co3+.