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Quantification of sampled blood volume is important in rare circulating cell detection. A sensitive double-clad fiber probe is utilized for in situ blood flow velocity measurements and cell size analysis by means of two-photon excited fluorescence correlation spectroscopy (FCS). The ability to measure flow velocities of fluorescently labeled cells in whole blood has been demonstrated. By simultaneously monitoring the distinct fluorescence signals from the labeled cells and fluorescent microspheres with known size, the flow velocity can be calibrated in real time and the average cell size can be calculated. The measured flow velocities agree with the theoretical estimation. However, the measured cell size is larger than the typical size, which is due to the comparable size between the cell and the probe area. Translational FCS measurements on particles of different sizes are conducted to confirm the particle size effect on the measured transit times.