The　dynamic　characteristics　of　simply　supported　pyramidal　truss　core　sandwich　beam　are　investigated.　The　nonlinear　governing　equation　of　motion　for　the　beam　is　obtained　by　using　a　Zig-Zag　theory.　The　averaged　equations　of　the　beam　with　primary,　subharmonic,　and　superharmonic　resonances　are　derived　by　using　the　method　of　multiple　scales　and　then　the　corresponding　frequency　response　equations　are　obtained.　The　influences　of　strut　radius　and　core　height　on　the　linear　natural　frequencies　and　hardening　behaviors　of　the　beam　are　studied.　It　is　illustrated　that　the　first-order　natural　frequency　decreases　continuously　and　the　second-order　and　third-order　natural　frequencies　initially　increase　and　then　decrease　with　the　increase　of　strut　radius,　and　the　first　three　natural　frequencies　all　increase　with　the　rise　of　the　core　height.　Furthermore,　the　results　indicate　that　the　hardening　behaviors　of　the　beam　become　weaker　with　the　increase　of　the　rise　of　strut　radius　and　core　height.　The　mechanisms　of　variations　in　hardening　behavior　of　the　sandwich　beam　with　the　three　types　of　resonances　are　detailed　and　discussed.