Natural rubber nanocomposites filled with hybrid fillers of multi-walled carbon nanotubes (CNTs) and carbon black (CB) were prepared. CNTs were ultrasonically modified in mixture of hydrogen peroxide (H

2

O

2

) and distilled water (H

2

O). The functional groups on the surface of CNTs

changes in nanotube structure and morphology were characterized by Fourier transform infrared spectroscopy (FT-IR)

Raman Spectroscopy

and transmission electron microscopy (TEM). It shows that hydroxyl (OH·) is successfully introduced. The surface defects of modified CNTs were obviously higher than those of original CNTs

and the degree of agglomeration was greatly reduced. Thermal conductivity of the composites was tested by protection heat flow meter method. Compared with unmodified CNTs/CB filling system

the thermal conductivity of hybrid composites is improved by an average of 5.8% with 1.5 phr (phr is parts per hundred rubber) of hydroxyl CNTs and 40 phr of CB filled. A three-dimensional heat conduction network composed of hydroxyl CNTs and CB

as observed by TEM

contributes to the good properties. Thermal conductivity of the hybrid composites increases as temperature rises. T

he mechanical properties of hybrid composites are also good with hydroxyl CNTs filled nanocomposites; the tensile strength

100% and 300% tensile stress are improved by 10.1%

22.4% and 26.2% respectively.