An optical fiber laser-Doppler system has ben developed to detect both the laser-induced-fluorescence (LIF) signal and the laser-Doppler signal of phytoplankton traversing through a measurement volume. From the two measurement, the size distribution, concentration, biomass, and the amount of chlorophyll present can be determined. Our apparatus have demonstrated successfully that it can resolve distinctly the size distribution of two species of phytoplankton (Chlorella and Scenedesmus). For phytoplankton, these two species do not have mark differences in size, one being less than 10 μm while the other is approximately 25 μm, and yet our measurement gave very satisfactory results. The result obtained clearly indicates that our optical system can produce accurate and liable size measurement and is suitable to be further develop into a practical in situ field instrument.