The thermal and electrical conductivities in nanocomposites of single walled carbon nanotubes (SWNT) and polyethylene (PE) are investigated in terms of SWNT loading, the degree of PE crystallinity, and the PE alignment. Isotropic SWNT/PE nanocomposites show a significant increase in thermal conductivity with increasing SWNT loading, having 1.8 and 3.5 W/mK at a SWNT volume fraction of phi similar to 0.2 in low-density PE (LDPE) and high-density PE (HDPE), respectively. This increase in SWNT/HDPE is more than additive and suggests a reduction of the interfacial thermal resistance. Fitting the thermal conductivity data of the SWNT/HDPE nanocomposites with two models indicates that the thermal conductivity relies on a percolating SWNT network. Oriented SWNT/HDPE nanocomposites exhibit higher thermal conductivities, which are attributed primarily to the aligned PE matrix.