In　order　to　obtain　the　theoretical　equations　of　in-plane　modes　for　cables　considering　bending　stiffness　and　sag　effect,　the　equations　of　motion　for　inclined　cable　considered　bending　stiffness　is　derived　in　the　local　coordinate　system　first.　Then,　based　on　the　assumption　of　flat-sag　cable　and　the　initial　shape　function　of　cable　suggested　by　Irvine,　the　theoretical　calculation　equations　for　in-plane　natural　frequencies　and　modal　shapes　of　flat-sag　inclined　cables　are　derived.　Compared　with　the　exact　results　obtained　by　finite　difference　method,　the　proposed　formulas　can　be　reliably　applied　to　calculate　the　in-plane　frequencies　and　mode　shapes　of　natural　vibration,　both　for　horizontal　cable　and　inclined　cable.