During the past decade, research on carbon nanotubes has revealed potential advances in thermal engineering applications. The present study investigates the radiative absorption and reflection of vertically aligned carbon nanotubes (VACNTs) in the broad spectrum from the near-infrared to far-infrared regions. The optical constants of VACNT are modeled based on the dielectric function of graphite and an effective medium approach that treats the CNT film as a homogenized medium. Calculated radiative properties show characteristics of near-unity index matching and high absorptance up to around 20 μm wavelength. The packing density and degree of alignment are shown to affect the predicted radiative properties. The Brewster angle and penetration depth of VACNTs are examined in the infrared spectrum. The radiative properties for VACNT thin films are also evaluated, showing some reduction of absorptance in the near-infrared due to transmission for film thicknesses less than 50 μm. This study provides a better understanding of the infrared behavior of VACNT and may guide the design for its applications in energy harvesting, space-borne detectors, and stealth technology.