© 2019 The Japan Society of Applied Physics. We reveal the characteristics of Brownian motion in nanoparticle clusters formed by the optical force fields at a solid-liquid interface based on the trajectory analysis from the microscopy movie data. The characteristics of stochastic motion depend not only on the laser power but also on the location in the clusters. The particles at the focal points exhibit smaller displacements, compared to the surrounding particles that are nevertheless trapped. The nominal diffusion coefficient is larger than the bulk at sufficiently small time scale for sufficiently high laser powers, although it decreases as a function of time because of the confinement. Decomposition of radial and circumferential components of displacements reveals the anisotropy as well as non-uniformity of the dynamics.