The core-shell structured PS/SiO2 composite nanospheres were synthesized on the basis of a modified Stober method. The mechanical properties of monodisperse nanospheres were characterized with nanoindentation on the basis of the atomic force microscopy (AFM). The surface morphologies of PS/SiO2 composite nanospheres was scanned with the tapping mode of AFM, and the force-distance curves were measured with the contact mode of AFM. Different contact model's were compared for the analyses of experimental data. The-elastic moduli of PS/SiO2 composite nanosphere (4-40 GPa) and PS nanosphere (similar to 3.4 GPa) were obtained with the Heitz and Johnson-Kendall-Roberts (JKR) models, respectively, and the JKR model was proven to be more appropriate for Calculating the elastic modulus of PS/SiO2 nanospheres. The elastic modulus of SiO2 Shell gradually approached a constant value (similar to 46 GP-a) with the increase of SiO2 shell thickness. A core-shell model was proposed for describing the relationship between PS/SiO2 composite nanosphere's elastic Modulus and shell thickness. The mechanical properties of the composite-nanospheres were reasonably explained on the basis of the growth mechanism of PS/SiO2 composite nanospheres, in particular the SiO2 shell's formation process. Available research data of PS/SiO2 composite nanospheres in this work can provide valuable guidance for their effective application in surface engineering, micro/nanomanufacturing, lubrication, and so on.