Three dimensional cell manipulation using two inclined fibers with lensed tips is demonstrated. For the first time, the relationship between optical forces and cell positions is experimentally characterized in such an optical fiber trap, which exhibits a good linearity in the vicinity of the equilibrium. The system is capable of being a force sensor with a resolution of around 1 pN/μm. The spring constant is found to be dependent on the cell's shape: a prolate cell entails a larger spring constant than that of a round cell with a similar size. Numerical analysis is carried out by using a modified ray optics model with a spheroidal object in the trap. The spring constant obtained from the analysis also depends on the shape of a cell, which agrees with the experimental results. The fiber optical tweezers have great potential for threedimensional manipulation and force measurement of cells.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.