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We propose a physical model to calculate the trapping force on a nanoparticle trapped by the system using the combination of a near-field scanning optical microscope (NSOM) probe and an atomic force microscope (AFM) metallic probe. Such a near-field trap is produced by evanescent illumination from the NSOM probe and light scattering at the tip of the AFM metallic probe. By using the Maxwell stress tensor through the electric field distribution obtained with the three-dimensional finite difference time domain (3-D FDTD) method, the dependence of the trapping force on the system parameters is discussed, and trapping properties including near-field distribution, trapping position, and the role of other forces versus trapping force are revealed. The results indicate that a particle down to tens of nanometers in size can be trapped toward the tip of an AFM probe with a lower laser intensity (∼1040 W/mm2) than that required by conventional optical manipulators (∼105 W/mm2).