This letter presents a method for measuring free-space transmittance to study the dielectric property of a thin and low-dielectric film in the millimeter wave region. We found that multireflection keeps the height of the repeated peaks of the transmittance almost constant and that their widths quickly narrow with increase in the incident angle. We further show how the dielectric constant is set for a 20-mum-thick silicon dioxide film on a 700-mum-thick silicon substrate near the electromagnetic waves of frequency 65 GHz. (C) 2004 American Institute of Physics.