Optical trapping of micron-size dielectric particles in a dual-fiber tip configuration is presented. Trap oscillation and suspension flow experiments are performed to investigate the linearity of the optical forces. These measurements are completed by standard methods such as Boltzmann statistics or power spectra evaluation. Strong trapping efficiencies of 0.25 and 1.8 pN·μm−11.8 pN·μm−1 have been found in the axial and transverse directions, respectively. The values obtained by the different approaches are in good agreement. The measurements show that the optical trapping potential is harmonic over the experimentally attainable distances, i.e., 2.5 and 0.6 μm in the transverse and axial directions, respectively.