Nanoparticles of barium carbonate/oxalate have been synthesized in situ from barium nitrate and ammonium carbonate/oxalate by reverse micelle method. These particles were characterized using transmission electron microscope (TEM), fourier transform infrared (FTIR) and x-ray diffraction (XRD). The results indicate that rod-like barium carbonate (BaCO3) and spherical barium oxalate (BaC2O4) nanoparticles can be fabricated with high crystallinity.