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To develop a new class of composites with adequately high thermal conductivity and suitably controlled dielectric constant for electronic packages and printed circuit board applications, polymer composites are prepared with microsized Al2O3 particle as filler having an average particle size of 80-100 μm. Epoxy and polypropylene (PP) are chosen as matrix materials for this study. Fabrication of epoxy-based composite is done by hand lay-up technique and its counterpart PP-based composite are fabricated by compression molding technique with filler content ranging from 2.5-25 vol%. Effects of filler loading on various thermal properties like effective thermal conductivity ( keff), glass transition temperature ( Tg), coefficient of thermal expansion (CTE) and electrical property like dielectric constant (εc) of composites are investigated experimentally. In addition, physical properties like density and void fraction of the composites along with there morphological features are also studied. The experimental findings obtained under controlled laboratory conditions are interpreted using appropriate theoretical models. Results show that with addition of 25 vol% of Al2O3, keff of epoxy and PP improve by 482% and 498% respectively, Tg of epoxy increases from 98°C to 116°C and that of PP increases from −14.9°C to 3.4°C. For maximum filler loading of 25 vol% the CTE decreases by 14.8% and 26.4% for epoxy and PP respectively whereas the dielectric constants of the composites get suitably controlled simultaneously. POLYM. COMPOS., 36:102-112, 2015. © 2014 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]