We experimentally demonstrate the tunable thermal property of a magnetically polarizable nanofluid that consists of a colloidal suspension of magnetite nanoparticles with average diameter of 6.7nm. Controlling the linear aggregation length from nano- to micron scales, the thermal conductivity (TC) of the nanofluid has been enhanced up to 216%, using 4.5vol% of nanoparticles. Repeated magnetic cycling shows that the TC enhancement is reversible. It has been confirmed that the large enhancement in TC is due to the efficient transport of heat through percolating nanoparticle paths. Our findings offer promising applications in "smart" cooling devices.