Velocities of magnetite particle crystals in heavy water-based ferrofluid have been studied by neutron spin echo PONTA in JRR-3M at the Bragg positions. Moving and rotating magnetite particle crystals in ferrofluid bring Doppler shift of neutron velocity at Bragg reflection though the scattering is elastic. This change of velocity is firstly measured by neutron spin echo (NSE) spectrometer as the function of Larmor precessions. Since the structure factor of (311) Bragg reflection is mainly nuclear scattering, the decay of NSE signal is interpreted as Doppler velocity shift due to Brownian motion of magnetite crystals, such as translational and rotational motions. The spin echo signals of (311) Bragg reflection show rather simple decay up to the number of Larmor precession, 967 turns. The estimated velocity distribution is 1.5+/-0.2 and 1.2+/-0.1m/s at 295 and 317K, respectively. While the spin echo signals of magnetic (111) reflection show spin flip scattering. The experiment of small angle scattering on the ferrofluid with wt% of magnetite from 1.3% to 40.5% concludes that the particle radius is 4.2+/-0.2nm, the cluster radius is 17.8+/-0.2nm and the neighboring average distance is around 11.3+/-1.1nm at 298K.