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The elasticity of an individual polymer nanoparticle may be greatly different from that of the bulk one. Understanding the properties of individual particles such as elasticity and deformation under exteal forces is of great importance in controlling the final structures and functions of bulk materials. We study the compression properties of single polyethylenimine (PEI) particles using vibrating scanning polarization force microscopy. By controllably imaging PEI particles at different vibration amplitude set-point values, it is demonstrated that we can compress the single PEI nanoparticle with an atomic force microscopy tip in different loads. Based on the force-height and force-strain curves obtained, Youngs moduli of PEI (5-160 Mpa) in three force regions are estimated according to the Hertz model. The results indicate that PEI has excellent elasticity, which may contribute to its high efficiency as vectors in gene transfection.