In the present study we have successfully synthesized cobalt and cobalt oxide NPs in ethanol and double distilled water respectively. Structural and optical characterizations have been performed by X-ray diffraction and UV-Visible absorption spectroscopy respectively. Magnetic characterization has been performed by vibrating sample magnetometer (VSM). Particle shape and size have been estimated by transmission electron microscopy (TEM). We have studied the effect of cobalt and cobalt oxide NPs on mitotic division of meristamatic roots of Sesbania cannabina. It has been found that cobalt oxide nanoparticles were more toxic than cobalt nanoparticles and both induced various types of chromosomal aberrations in meristamatic root tip cells of sesbania cannabina.