Silica nanoparticles (SNPs) are particles of silica which are in the range of 1 to 100 nm in size. In the present study, native silica nanoparticles (SNPs) have been prepared chemically using cetyltrimethyl ammonium bromide (CTAB) and sodium silicate as precursors. The prepared native SNPs were organically modified using the trioctylphosphine oxide (Cyanex921) to yield the modified CY-SNPs particles. The synthesized SNPs and CY-SNPs were characterized by using Scanning Electron Microscope (SEM) micrographs that showed spherical particles with an average size of 35 nm. The elemental analysis was performed using the Energy Dispersive Spectrum (EDS). For the native SNPs, The peaks recorded around 1.9 Kv and 0.5 Kv are the binding energies corresponding to Si and O, respectively. For the modified CY-SNPs, an additional peak around 2.1 Kv corresponding to phosphorous from the cyanex921 moiety was observed. The powder X-Ray Diffraction (XRD) analysis of the material further confirmed the crystallinity of the SNPs as evidenced by the diffraction pattern at broad peak centered at 2ϴ=20o which indicates that the sample is amorphous. Fourier Transform Infra -Red Spectroscopy (FTIR) spectra indicated the loading of cyanex921 to the surface of the native SNPs. The antimicrobial activities of native and phosphine modified SNPs were assessed against Gram-positive and Gram-negative microorganisms using the minimum inhibitory concentration (MIC) studies. The results obtained showed that CY-SNPs have potential antibacterial effects against the gram positive microorganisms, viz. methicillin resistant staphylococcus aureus (MRSA) and the gram negative microorganisms viz. E.coli, klebsiella. Spp and psuedomonas.spp and the MIC corresponding to each type has been determined. The native silica nanoparticles have not shown any antibacterial effect against the four bacteria strains previously mentioned over concentrations 0.10-5.0 mg/mL. The mechanism of the antibacterial activity of the proposed phosphine modified silica nanoparticles was discussed. The present study can generally approve the qualification of caynex921modified silica nanoparticles for the use as antibacterial agents.