We report the spectral and optical limiting properties of two kinds of HA P nanoparticles (pure HA P and HA P@SiO 2 core-shell nanoparticles) that were wetchemically prepared and coated with silica by means of Stober method. The size, framework substitution and silica coating formation of the resultant particles were confirmed by Transmission Electron Microscopy, X-ray diffraction, Fourier Transform Infrared Spectroscopy and Thermogravimetric analysis. The nonlinear optical limiting properties of HA P and HA P@SiO 2 nanoparticles suspended in ethylene glycol is studied at 532 nm using 5 ns Nd:YAG laser pulses. Observations indicate that effective threephoton absorption is found to be responsible for strong optical limiting characteristics in these nanoparticles and it is seen that the optical nonlinearity is enhanced in core- shell structures as compared with single counterparts. The nonlinear optical parameters calculated from the data show that these materials are efficient optical limiters. © Springer Science+Business Media, LLC 2011.