This paper describes the development of a novel dry film-type dielectric material with low loss tangent (Df) and the demonstration of a low-loss filter substrate using the dielectric material for high-frequency transmission applications. This paper also presents the evaluation results of the small filter characteristics of the substrate in the 28 GHz and 39 GHz 5G millimeter-wave (mmWave) band. We have recently developed a dry film dielectric material with outstanding electrical properties and excellent mechanical properties (Material P). This new material is based on polyphenylene ether (PPE) that has extremely low Df. PPE is commonly known as a thermoplastic polymer, henceforth a new chemical design was applied to modify the polymer structure into a thermosetting polymer. The new dielectric material can be processed at a low temperature about 200 0C and is compatible to the standard substrate manufacturing processes, such as semi additive process (SAP). Material characterization revealed that Dk/Df of Material P is 3.1/0.0013 at 10 GHz, and glass transition temperature (Tg) is 200 0C. In this work, RF filter performance of the Material P was characterized to demonstrate the benefit of the low loss material. As the reference, the performance of epoxy dielectric was additionally characterized and compared. Electrical characterization of the filter structures showed low transmission losses < 1.0 dB at 28 GHz and < 0.8 dB at 39 GHz with Material P, verifying applicability of the material for high frequency applications.