A nanocomposite that incorporates cobalt oxide(Co_3O_4) and nanodiamond(ND) can present both high magnetism(Co_3O_4) and high hardness(ND). ND particles have potential applications in a variety of fields such as protein immobilization, biosensors, therapeutic molecule delivery and bio-imaging. However,limited information is available in literature on the in-situ synthesis of biocompatible magnetic materials and also on their potential biotoxicity as a result of their entry into environmental compartments and subsequent interaction with biota. In this work, a new kind of bio-compatible magnetic material –carboxylated nanodiamond(c ND) and Co_3O_4 was synthesized to obtain the c ND-Co_3O_4 nanocomposite.The synthesis procedure involved in-situ and chemical reduction of cobalt chloride(CoCl_(26)H_2O) and sodium borohydrate(NaBH_4). The synthesized cND-Co_3O_4 nanocomposite was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The cyto-genotoxicity of the synthesized nanocomposite material was studied by using onion(Allium cepa L.) as a test model with concentrations of 5, 10 and 20 μg/ml. The study was also extended to cND and Co_3O_4 materials for comparison purpose. Co_3O_4 and cND exhibited their contrasting effects on mitosis and other cyto-genotoxic indices studied herein. This work provided fundamental data on the synthesis and the bio-toxicity of the c ND-Co_3O_4 nanocomposite, which, in turn, can help to expand their multidisciplinary applications. A nanocomposite that incorporates cobalt oxide (Co_3O_4) and nanodiamond (ND) can present both high magnetism (Co_3O_4) and high hardness (ND). ND particles have potential applications in a variety of fields such as protein immobilization, biosensors, therapeutic molecule delivery and However, limited information is available in literature on the in-situ synthesis of biocompatible magnetic materials and also on their potential biotoxicity as a result of their entry into environmental compartments and subsequent interaction with biota. In this work, a new kind of bio-compatible magnetic material-carboxylated nanodiamond (cND) and Co_3O_4 was synthesized to obtain the c ND-Co_3O_4 nanocomposite. The synthesis procedure involved in-situ and chemical reduction of cobalt chloride (CoCl_ (26) H_2O) and sodium borohydrate NaBH_4). The synthesized cND-Co_3O_4 nanocomposite was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and X-ray ph otoelectron spectroscopy. The cyto-genotoxicity of the synthesized nanocomposite material was studied by using onion (Allium cepa L.) as a test model with concentrations of 5, 10 and 20 μg / ml. The study was also extended to cND and Co_3O_4 materials for comparison purpose. Co_3O_4 and cND images their contrasting effects on mitosis and other cyto-genotoxic indices studied herein. This work provides fundamental data on the synthesis and the bio-toxicity of the c ND-Co_3O_4 nanocomposite, which, in turn, can help to expand their multidisciplinary applications.