Low‐loss Sr2Ti9O20 (STO)/Butyl Rubber composites have been prepared by mixing and hot pressing. X‐ray diffraction (XRD) and energy dispersive spectroscopy (EDS) are employed to investigate the phases contained in the STO filler, and the microstructural characteristics of the composites are discovered using scanning electron microscopy (SEM). The STO/BR composites demonstrate a high relative permittivity of 10.98 and a low loss of 0.003 at 10 GHz, corresponding to an optimum filler volume fraction of 45%. In addition, a low CTE of 50.72 ppm oC‐1 and a high thermal conductivity of 0.9603 Wm‐1K‐1 have been found for the optimum composition, with an acceptable moisture absorption of 0.071%. The obtained composites can be employed to design miniaturized, flexible RF substrate applications.This article is protected by copyright. All rights reserved.