Traditionally,　only　the　intensity　measure　of　wind　speed　is　used　to　perform　the　failure　analysis　of　transmission　lines　(TLs).　However,　during　typhoon　landing,　both　the　wind　and　rainfall　effects　on　the　failure　probability　of　TLs　should　be　considered.　To　fill　the　gap　of　rainfall　amount　data　measured　with　long　sampling　periods,　this　study　presents　a　conversion　approach　of　rain　intensities　with　various　time　resolutions　based　on　the　meteorological　data　of　1336　stations.　Then,　the　12-hour　rain　intensity　samples　recorded　over　60　years　are　converted　into　10-min　rain　intensity　samples,　followed　by　the　establishment　of　the　marginal　probability　distributions　of　yearly　extreme　wind　speed　and　rain　intensity.　Furthermore,　according　to　the　theory　of　Copula　function,　the　joint　probability　distribution　of　wind　speed　and　rain　intensity　is　proposed　using　the　Clayton　copula　which　has　the　best　goodness　of　fit.　Finally,　a　case　study　is　performed　and　the　failure　probabilities　of　a　TL　under　combined　wind　and　rain　loads　are　calculated.　The　failure　probability　of　the　employed　TL　under　combined　wind　and　rain　loads　has　reached　6.19%,　and　the　value　is　only　2.88%　if　ignoring　the　rainfall　effect,　demonstrating　that　rainfall　has　significant　influence　on　the　structural　safety　of　a　TL.