In many industrial processes, surfactants are used to control the dispersion and coagulation of particles in aqueous solutions. Especially, surfactants have critical effects in flotation separation. To investigate the effects of surfactants to the surface force between bubble and particle, which is critical for the particle recovery in flotation, we conducted direct measurements of the interaction forces between a spherical silica particle and an air bubble in dodecylamine hydrochloride (DAH) solution using atomic force microscope. Interaction forces were measured between the particle attached on the top of AFM probe and bubble generated on the flat surface.<BR>When a hydrophilic silica particle was used, electrostatic force was observed at large separation distance on approach of the silica and the bubble. Then long-range attractive force acted between the surfaces even when DAH concentration was low. The range of attractive force includes the penetration distance of particle into the bubble. The relation between the floatability of silica particle in batch column flotation and the interaction force was discussed.<BR>On the other hand, when the hydrophobized silica was used, the range of attractive force became short with the increase of DAH concentration. The repulsive force before attraction became very strong at high DAH concentration. The origin of the strong repulsive force is assumed to be the steric force between adsorbed dodecylammonium ions. These interaction forces cause the depression of the particle in flotation.