Glass　fiber-reinforced　polymer-matrix　(GFRP)　composites　are　used　to　manufacture　devices　such　as　radomes,　electrical　insulators,　and　radar-absorbing　structures,　many　of　which　are　used　in　applications　with　strict　mechanical　and　dielectric　property　requirements.　As　such,　designs　and　properties　of　GFRP　composite　materials　should　be　regularly　improved　to　guarantee　their　safe　operation　and　meet　their　constantly　evolving　application　requirements.　In　this　study,　glass　fiber　(GF)　particles　and　epoxy　resin　128　(EP-128)　were　used　to　manufacture　high-performance　EP/GF-X　composites　with　GF　particle　contents　ranging　from　5%　to　40%.　An　investigation　into　their　mechanical　and　dielectric　properties　revealed　that　EP/GF-20　composite　specimens　presented　the　best　mechanical　performance　with　their　bending　strength　and　modulus　reaching　159.68　MPa　and　6521.92　MPa,　respectively,　and　an　average　dielectric　constant　of　4.862　within　the　10-10(5)　Hz　frequency　range.　A　theoretical　analysis　of　the　EP/GF-X　composite　specimens＇　dielectric　constants　using　classic　dielectric　models　based　on　the　Lichtenecker,　Bruggeman,　Jaysundere-Smith,　and　Maxwell-Garnett　rule-of-mixture　equations　indicated　that　the　dielectric　model　based　on　the　Bruggeman　rule-of-mixture　equation　provided　the　dielectric　constants　closest　to　the　measured　values　with　a　maximum　deviation　of　less　than　-0.321%　for　EP/GF-40　composites.　This　study　provides　a　feasible　strategy　to　control　the　mechanical　and　dielectric　properties　of　GFRP　composites.