A　novel　method　to　calculate　the　dielectric　constant　(epsilon(r)&　PRIME;)　of　three-dimensional　orthogonal　woven　glass　fiber-reinforced　polymer-matrix　(3DOW-GFRP)　composites　in　the　terahertz　(THz)　frequency　range　is　developed　and　experimentally　verified　using　the　THz　time-domain　spectroscopy　(THz-TDS).　The　dielectric　anisotropy　is　demonstrated　through　simulation　by　considering　a　single　propagation　direction　of　the　THz　waves　and　three　different　fiber　orientations　in　unidirectional　GFRP　composites.　Simulation　results　are　compared　to　those　obtained　from　the　classic　rule-of　mixture　equations　to　determine　the　representative　equations　for　the　three　different　cases　of　fiber　orientations　in　the　(x,y,z)-coordinate　system　and　a　new　model　is　developed　to　calculate　epsilon(r)&　PRIME;　of　3DOW-GFRP　composites　based　on　electromagnetic　modeling　principles.　As　opposed　to　previously　reported　lumped　circuit　models,　our　model　addresses　the　issue　of　orthogonality　of　fiber　and　considers　the　shape　and　spatial　disposition　of　the　composite　with　respect　to　the　polarization　direction　of　the　THz　waves　for　accurate　determination　of　epsilon(r)&　PRIME;.　A　comparison　between　the　measured　and　calculated　epsilon(r)&　PRIME;　indicates　that　the　proposed　method　is　highly　accurate　with　&　LE;　2.68%　maximum　error,　while　the　latter　reaches　7.82%　for　the　classic　rule-of-mixture　equations.　This　method　is　potentially　useful　for　the　design　of　3DOW-GFRP　with　desired　epsilon(r)&　PRIME;　for　applications　such　as　electromagnetic　shielding　and　electrostatic　discharging　structures.