A facile strategy to prepare water-soluble multiwalled carbon nanotubes (MWNTs) by two steps of gamma radiation is developed. First, MWNTs are irradiated in ethanol. The radiolysis of ethanol produces many active species such as (CHCH2OH)-C-center dot and (CH)-C-center dot(CH3)OH which react with C=C bonds on the surface of MWNTs. Afterward, poly(acrylic acid) (PAA) is covalently grafted to the surface of modified MWNTs by radiation in the presence of acrylic acid. FT-IR, H-1 NMR, and TGA results showed that PAA chains were successfully grafted onto the surface of MWNTs. The TEM images also provide direct evidence of the formation of a core-shell structure, and the external diameter of resultant MWNTs is increased remarkably. The PAA-g-MWNTs have very good Solubility in water and other polar solvents. This two-step grafting approach may also be employed introducing other functional polymer chains onto MWNTs.