Un-doped and Co-doped Zn O nanoparticles(NPs) with different weight ratios(0.5, 1.0, 1.5, and 2.0 wt% of Co)were synthesized by a facile and rapid microwave-assisted combustion method using urea as a fuel. The prepared NPs were characterized by X-ray diffraction(XRD), high resolution scanning electron microscopy(HR-SEM),energy dispersive X-ray analysis(EDX), diffuse reflectance spectroscopy(DRS), photoluminescence(PL)spectroscopy and vibrating sample magnetometry(VSM). XRD patterns refined by the Rietveld method indicated that Co-doped Zn O had a single pure phase with wurtzite structure suggesting that Co2 t ions would occupy Zn2 t ionic sites within the Zn O crystal lattice. Interestingly, the morphology was found to convert substantially from grains to nanoparticles with close-packed periodic array of hexagonal-like shape and then into randomly distributed spherical NPs with the variation of Co-content. The optical band gap estimated using DRS was found to be red-shifted from 3.22 e V for the un-doped Zn O NPs then decrease up to 2.88 e V with increasing Co-content. PL spectra showed a strong green emission band thus confirming the formation of pure single Zn O phase. Magnetic studies showed that Co-doped Zn O NPs exhibited room temperature ferromagnetism(RTFM) and that the saturation magnetization attained a maximum value of2.203 103emu/g for the highest Co-content. The antibacterial studies performed against a set of bacterial strains showed that the 2.0 wt% Co-doped Zn O NPs possessed a greater antibacterial effect. Un-doped and Co-doped Zn O nanoparticles (NPs) with different weight ratios (0.5, 1.0, 1.5, and 2.0 wt% of Co) were synthesized by a facile and rapid microwave-assisted combustion method using urea as a fuel. prepared NPs were characterized by X-ray diffraction (XRD), high resolution scanning electron microscopy (HR-SEM), energy dispersive X-ray analysis (EDX), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy and vibrating sample magnetometry (VSM). XRD patterns refined by the Rietveld method indicated that Co-doped Zn O had a single pure phase with wurtzite structure suggesting that Co 2 t ions would occupy Zn 2 t ionic sites within the Zn O crystal lattice. Interestingly, the morphology was found to convert substantially from grains to nanoparticles with close-packed periodic array of hexagonal-like shapes and then into randomly distributed spherical NPs with the variation of Co-content. The optical band gap estimated using DRS was found to be red-shifted from 3.22 e V for the un-doped Zn O NPs then decrease up to 2.88 e V with increasing Co-content. PL spectra showed a strong green emission band thus confirming the formation of pure single Zn O phase. NPs showed room temperature ferromagnetism (RTFM) and that the saturation magnetization attained a maximum value of 2.203 103 emu / g for the highest Co-content. The antibacterial studies performed against a set of bacterial strains showed that the the 2.0 wt% Co-doped Zn O NPs possessed a greater antibacterial effect.