Wood pulp cellulose nanofibrils (CNF) thin film is a novel recyclable and biodegradable material. We investigated the microwave dielectric properties of the epoxy coated-CNF thin film for potential broad applications in flexible high speed electronics. The characterizations of dielectric properties were carried out in a frequency range of 1-10 GHz. The dielectric constant and loss tangent were extracted by using measurements of microstrip transmission lines that were built on the epoxy coated-CNF film and combined with Agilent Advanced Design System simulations. The RF property for the epoxy coated-CNF was compared to that of commercial polyethylene terephthalate film (PET). With the comparable microwave properties of non-biodegradable PET film, our study suggests that the epoxy coated-CNF film is a suitable biodegradable and environment-friendly substrate for flexible microwave and other applications.