ZnO-BaO particles (4-10) nm have been synthesized by applying 40 KHz ultrasonic wave simultaneously with the co-precipitation process of Zn and Ba oxalate at 50 0C. DTA curves of the co-precipitate were carried out and the decomposition temperature was determined. Mixed oxide obtained from thermal decomposition of Zn2 Ba (C2O4)2.2H2O in air has been investigated by TEM, XRD, IR and UV-Vis measurements. The results revealed that the obtained particle consists of a core of highly deformed Ba contaminated ZnO nano-crystals. The ZnO crystalline core is coated by Ba rich amorphous layer which prevents its growth. Decreasing concentration of ZnCl2 in the precipitation solution from 0.1M to .05 M leads to decrease the size of the particle from (10-20) nm to(4-10) nm, in average, increases UV cut-off, the cut-off tail and rounded of the shape of the particle.