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　10–105　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.　Highlights:　Glass　fiber　(GF)　particles　and　epoxy　128　(EP-128)　resin　were　used　to　manufacture　high-performance　glass　fiber-reinforced　epoxy　(EP/GF-X)　composites.　The　composite　manufacturing　process　was　carefully　controlled　to　ensure　uniform　distribution　and　orientation　of　GF　particles　in　the　EP-128　resin.　The　mechanical　and　dielectric　properties　of　the　manufactured　EP/GF-X　composites　were　effectively　tailored　by　controlling　the　content　and　distribution　of　GF　particles　in　the　EP-128　resin　matrix.　The　dielectric　constants　of　the　EP/GF-X　composites　were　measured　experimentally　and　predicted　using　various　theoretical　dielectric　models.　The　Bruggeman　model　provided　the　dielectric　constants　of　the　EP/GF-X　composites　closest　to　the　measured　values.　©　2023　Society　of　Plastics　Engineers.