S-parameters　are　widely　used　to　detail　the　scattering　parameters　of　radio　frequency　(RF)　components　and　microwave　circuit　modules.　The　vector　network　analyzer　(VNA)　is　the　most　commonly　used　device　for　measuring　S-parameters.　Given　the　multiple　frequency　points,　complex　values,　and　intricate　uncertainty　propagation　involved,　accurately　assessing　the　uncertainty　of　S-parameter　measurements　is　difficult.　In　this　study,　we　proposed　a　new　method　for　assessing　S-parameter　uncertainty　based　on　the　covariance　matrices,　tracing　back　to　the　nominal　uncertainty　of　calibration　standards.　First,　we　analyzed　the　relevant　theory　of　uncertainty　assessment　using　covariance　matrices　and　subsequently　deduced　the　mechanism　of　Type　B　uncertainty　propagating　from　calibration　standards　to　error　model　coefficients　and　S-parameter　measurements　to　evaluate　Type　B　measurement　uncertainty.　In　this　study,　a　novel　measurement　system　was　constructed　for　measuring　grounded　coplanar　waveguides　by　using　a　VNA　and　calibration　standards　with　8-　and　12-error　models.　Initially,　the　model　assessed　the　Type　B　uncertainty　of　measuring　four　S-parameters　of　a　grounded　coplanar　waveguide.　Next,　the　VNA　calibrated　with　the　12-error　model　was　used　to　conduct　multiple　repeated　measurements　to　assess　the　Type　A　uncertainty　of　the　grounded　coplanar　waveguide.　Finally,　the　composite　uncertainty　was　constructed,　which　demonstrated　that　the　proposed　method　can　be　used　for　assessing　the　uncertainty　of　S-parameters.