A zero-crossing dynamic speckle method is proposed to determine the velocities of nanoparticles in nanofluids.A Gaussian laser beam is used to illuminate nanofluids in a pipe,and the dynamic speckles are detected by a spatially integrating detector with an aperture.The integrated speckle intensity signal is processed by a computer and the zero-crossing rate is counted.The velocity of the nanoparticles can be determined from its relationship to zero-crossing rate.The results show that the nanoparticles exhibit features of flowing nanofluids,and when the average velocity of the nanofluids is 53.4mm/s,the average velocity of the nanoparticles is 51.8 ± 5.1 mm/s.