An　investigation　on　the　nonlinear　vibration　behaviors　of　a　lattice　sandwich　beam　with　pyramidal　truss　core　and　functionally　graded　material　(FGM)　facesheets　under　thermo-mechanical　loads　is　performed　with　consideration　of　the　temperature-dependency　of　material　properties.　In　present　work,　an　improved　equivalent　methodology　is　proposed　to　determine　the　transverse　shear　modulus　of　pyramidal　core　subjected　to　a　non-uniform　thermal　field　across　thickness　direction　and　makes　lattice　truss　core　transformed　to　a　continuous　layer.　Finally,　the　lattice　sandwich　beam　is　modeled　as　a　three-layered　sandwich　structure.　The　facesheets　are　made　of　FGMs　with　mixture　of　ceramics　and　metals　in　a　pow-law　form.　The　outer　surfaces　of　facesheets　are　ceramic-rich,　while　the　inner　surfaces　are　metal-rich.　The　nonlinear　strain-displacement　relations　are　established　based　on　the　von　Karman　large　deflection　and　classical　beam　theory.　The　Lagrange　method　are　implemented　to　yield　equations　governing　the　free　and　forced　vibration　behaviors.　With　the　help　of　Newton-Raphson　iteration　technique　as　well　as　the　Newmark-beta　method,　the　nonlinear　time-independent　responses　of　the　sandwich　beam　under　thermal　environment　are　solved.　A　comprehensive　parameter　study　is　directed　to　address　the　effects　of　non-uniform　thermal　environment,　FMG　facesheets,　external　load,　and　lattice　core　on　the　nonlinear　vibrations　of　the　beams.