In order to reveal the nearfield distribution of electromagnetic field and the axial distribution of trapping force acting on nanoparticles located in nearfield optical tweezers,and to explore the trapping size,trapping location and trapping stability,the direct calculations of Maxwell stress tensor were solved numerically by using threedimensional finite difference time domain method without the limitation of dipole approximation.Calculations show that the particle with radius larger than the aperture is pushed away from the tapered metalcoated fiber probe,while it tends to be trapped in larger effective region as the radius becoming smaller.The nearfield optical probe can trap and manipulate a nanoparticle with tens of nanometres diameter to different positions with different distance from the probe tip.Results of the calculations are found to be helpful for the design and manifacturing of nearfield optical tweezers system.验室开放基金项目(No.2009EP012)和机器人技术与系统国家重点实验室开放研究项目(No.SKLRS20102D10)资助