The laser diode self-mixing interferometry can be employed to measure nano-particles and nano-fluids. The analytical expression of power density of the feedback self-mixing signal from a flowing Brownian motion system is obtained theoretically. It reveals that the power density is of the Voigt profile, which is a convolution of the Lorentzian and Gaussian line profiles. The characteristics of the self-mixing spectrum are studied numerically. It is found that the Doppler shift and the Gaussian term in the power density are determined by flow velocity while the Lorentzian term is determined by the Brownian motion of particles (i.e., the particle size). The study provides fundamentals for the measurement of nano-fluids.