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The dynamic property of ferrofluids under a dc-biased magnetic field was understood by the measurement of optical transmittance. The results show that immediately after the application of a magnetic field, the transmitted optical intensity decreases to a minimum and, then, increases until it becomes stable. From the microscopic images, they indicate that this effect is due to longitudinal aggregation of the magnetic nanoparticles, forming chains. When the chain-like structure was formed by the applied magnetic field, the optical transmittance was decreasing. As the chains became longer and the distance between chains is larger, the transmittance was increasing. Moreover, our experiment result shows that this dynamic process is influenced by a different applied magnetic field, sample concentration, and carrier fluid.