We propose a new method to measure two-dimensional particle size of nanoparticles in imagery based on the principle of dynamic light scattering, which is called the translation-rotation image-dynamic light-scattering (TR-IDLS) method. The nanoparticles in Brownian motion in the sample pool are illuminated by a focused polarized Gaussian beam; then, the horizontal and vertical polarized scattering signals of the nanoparticles are recorded. According to the time-correlation function applied to the fluctuation of scattered-light intensity in the two polarization directions, the distributions of translational- and rotational-diffusion coefficients of nanoparticles are calculated, and the aspect-ratio, equivalent-diameter, and thickness distributions are obtained. The spherical standard nanoparticles and mica-flake particles are measured by this method. The shape and equivalent diameter of the mica-flake particles obtained by scanning electron microscopy are compared with the experimental results obtained by the TR-IDLS method, thereby verifying the feasibility of the TR-IDLS method.