Ultrasmall (<5 nm) gamma-Fe2O3 nanocrystals with tunable loading density on the carbon nanotubes (CNTs) were solvothermally synthesized using bifunctional phenol as capping ligand and dually as surfactant that favored uniform attachment. With the interfacial charge transfer, the nanosize-dependently superparamagnetic gamma-Fe2O3 nanoparticles transform to weak ferromagnetism at room temperature.