In this study, we report the length dependence of thermal conductivity (k) of zinc blende-structured Zinc Selenide (ZnSe) and Zinc Telluride (ZnTe) for length scales between 10 nm and 10 μm using first-principles computations, based on density-functional theory. The k value of ZnSe is computed to decrease significantly from 22.9 W m⁻¹ K⁻¹ to 1.8 W m⁻¹ K⁻¹ as the length scale is diminished from 10 μm to 10 nm. The k value of ZnTe is also observed to decrease from 12.6 W m⁻¹ K⁻¹ to 1.2 W m⁻¹ K⁻¹ for the same decrease in length. We also measured the k of bulk ZnSe and ZnTe using the Frequency Domain Thermoreflectance (FDTR) technique and observed a good agreement between the FDTR measurements and first principles calculations for bulk ZnSe and ZnTe. Understanding the thermal conductivity reduction at the nanometer length scale provides an avenue to incorporate nanostructured ZnSe and ZnTe for thermoelectric applications.