Transparency and thermal conductivity of ultradrawn, ultrahigh-molecular-weight polyethylene films containing different contents of low-molecular-weight polyethylene wax (PEwax) are explored from experimental and theoretical viewpoints. It is shown that the addition of PEwax decreases light scattering in all directions, resulting from a reduction of defects while having little effect on crystallinity or chain orientation of ultradrawn films. In general, upon the addition of PEwax, the thermal conductivity of ultradrawn films increases with the highest conductivity being 47 (W m(-1) K-1) and subsequently decreases at higher concentrations. The thermal conductivity also depends on draw ratio and number-average molecular weight (Mn) of the films. A model is presented which correlates the thermal conductivity of the films with the draw ratio and Mn, enabling explanation of the experimental results. Hence, the thermal conductivity of ultradrawn polyethylene films can be predicted as a function of Mn and draw ratio.