Microscale multiphase flow system will be a promising methodology in micro- and nanoscale technology. The present study focused on investigation of nanoscale particle motion in a microchannel detected by a micro-resolution particle image velocimetry (micro-PIV). Magnetic particles of 350 nm in diameter in the microchannel were controlled by the magnetic field, yielding decelerating fluid flow. When the magnetic field was removed, particles were dispersed rapidly by the Brownian motion whose theoretical diffusion velocity was equal to one obtained by the present micro-PIV system.