Large amount of crude oil remains in the reservoir due to the poor sweep and displacement efficiency after displacing fluid injection. To remediate this effect, a thicker displacing fluid is used to reduce viscous fingering for a more stable flood front. A ferrofluid is a suitable candidate due to the tunable viscosity profile when subjected to a magnetic field 1. In this work, the ability of cobalt substituted magnetite ferrofluid to improve incremental recovery after waterflooding has been investigated via sand pack flooding. Prior to sand pack flooding, structural and magnetic properties of cobalt substituted magnetite nanoparticles were characterized via XRD, FESEM and VSM. Viscosity tests with field strength variation from 0 to 66.88 mT have shown a significant dependency of the ferrofluid's viscosity on the applied field strength. 6-fold increment of viscosity was recorded when magnetic field strength changes from 19.5 to 66.88 mT. During sand pack flooding, 7.20% of incremental oil was obtained with the ferrofluid injection, even without the presence of a magnetic field. When subjected to a magnetic field, 12.93% and 15.83% of the incremental oil was obtained at 19.5 and 66.88 mT, respectively. It is proven that increase of ferrofluid viscosity with magnetic field strength results in higher incremental recovery. Improved sweep and displacement efficiency has been achieved by injecting the ferrofluid into the oil reservoir. © 2019 Trans Tech Publications, Switzerland