By　incorporating　the　strain　gradient　elasticity　into　the　classical　Bernoulli-Euler　beam　and　Timoshenko　beam　models,　the　size-dependent　characteristics　of　wave　propagation　in　micro/nanobeams　is　studied.　The　formulations　of　dispersion　relation　are　explicitly　derived　for　both　strain　gradient　beam　models,　and　presented　for　different　material　length　scale　parameters　(MLSPs).　For　both　phenomenological　sizedependent　beam　models,　the　angular　frequency,　phase　velocity　and　group　velocity　increase　with　increasing　wave　number.　However,　the　velocity　ratios　approach　different　values　for　different　beam　models,　indicating　an　interesting　behavior　of　the　asymptotic　velocity　ratio.　The　present　theory　is　also　compared　with　the　nonlocal　continuum　beam　models.