There has been increasing interest in the use of magnetic fluids to manipulate diamagnetic particles in microfluidic devices. Current methods for diamagnetic-particle focusing in magnetic fluids require either a pair of repulsive magnets or a diamagnetic sheath flow. We demonstrate herein a tunable, sheathless focusing of diamagnetic particles in a microchannel ferrofluid flow with a single set of overhead permanent magnets. Particles are focused into a single stream near the bottom wall of a straight rectangular microchannel, where a magnetic-field minimum is formed as a result of the magnetization of the ferrofluid. This focusing can be readily switched off and on by removing and replacing the permanent magnets. More importantly, the particle-focusing position can be tuned by shifting the magnets with respect to the microchannel. We perform a systematic experimental study of the parametric effects of the fluid-particle-channel system on diamagnetic-particle focusing in terms of a defined particle-focusing effectiveness.