A series of powders of (Ba1-xLa2x/3)(Ti1-xNb4x/5)O3 (0.05≤x≤0.3) have been synthesized by solid-state reaction route, and the effects of the dopants on the microstructure of BaTiO3 were investigated by XRD, SEM, HTEM and EDS in the BaTiO3-La2O3-Nb2O5 ternary system. It was found that the secondary titanium-rich phase occurred for x ≤ 0.1, while Ba3La3Ti5Nb5O30 yields as secondary phase for x ≥ 0.2. In the whole range of 0.05 ≤ x ≤ 0.3, the crystal structure of BaTiO3 solid solutions is cubic system. There are two substitution modes in the system: the substitution mode of La ions dissolved into Ba sites is significant for x ≤ 0.2, but the substitution mode of Nb ions dissolved into Ti sites is significant for x>0.2. TEM observation and EDS analysis revealed that the concentration gradient of Nb existed and La almost distributed homogeneously in a grain. Thus, the dielectric materials composed of La-Nb-codoped BT are not fitted to form the core-shell structure for the higher dopants.