In this paper, the effect of multiwalled carbon nanotubes (MWCNTs) on the microstructure, rheological and thermal properties of the reinforced low density polyethylene/carbon fiber (LDPE/CF) composite was investigated. The dispersion state of single and hybrid fillers in the LDPE matrix was studied in detail by means of melt viscoelastic experiment together with field emission electron microscopy (FESEM) results, which revealed high dispersion degree of CF and CNTs. The obtained results demonstrated that the mixing process was adequate. It was shown that the incorporation of MWCNTs into LDPE/5%CF composite created a large surface area which can be established strong interfacial adhesion between the efficient hybrid filler networks and the matrix. Finally, thermal analysis of samples was studied by using TGA-DTG analysis. The incorporation of carbon fillers effectively improved the thermal stability.