We present a novel optical fiber dynamic light scattering measurement system for nanometer particle size. A multimode fiber probe is used to highly efficiently couple and deliver laser light, while a single-mode fiber probe is used to only receive single scattering light. This design not only improves the measuring ability of the system but also provides flexibility in choosing scattering angle for measurement. By using this system, four kinds of polystyrene nanoparticles are evaluated. Experimentation shows the measured sizes of the nanoparticles are well consistent with the nominal ones, verifying the feasibility of the system.