The trapping force on Rayleigh particles in an optical tweezers system with
an oil immersion objective is calculated by an electromagnetic model. The
results indicate that the stability of particles trapped will be affected b
y spherical aberration. which is caused by refractive difference between ob
jective oil and water solution, when the specimen manipulated is suspended
in a water solution. The trapping force and depth of potential well will de
crease and the minimum of laser power for ensuring the stability of particl
es trapped will increase with the enhanced trapping depth.