In this study, polymer matrix and carbon filler which are on the influence of different manufacturing conditions and type of polymer matrix were used for the fabrication of thermal conductive composite materials. The carbon material was used MWCNT (multi walled carbon nanotubes), the polymer matrix were used PP (polypropylene) and HDPE (high density polyethylene). For the fabrication of the composite materials, pristine MWCNT and ball-milled MWCNT, with altered lengths, were mixed with polymer matirx (0, 2, 4, and 6 wt. % MWCNT) using stirring device with an internal mixer. An attrition ball mill was used for the milling of MWCNT with different milling times (15, 30, 45, and 60 min). The analysis of the MWCNT particle length and change of powder properties were done by FE-SEM and nano raman spectroscopy. The thermal characteristic was analyzed by differential scanning calorimetry(DSC) and thermogravimetric analyzer(TGA). And, the thermal conductivity values of the PP/MWCNT and HDPE/MWCNT composite was analyzed by laser flash method. The effects of milling time on the thermal conductivity were evaluated.