The　propagation　behavior　of　Lamb　wave　in　a　magneto-electro-elastic　plate　in　magnetic　field　was　investigated.　The　stress　and　electric　displacement　and　magnetic　induction　intensity　in　the　plate　were　derived.　Then　the　phase　equations　of　symmertric　and　anti-symmertric　modes　of　Lamb　wave　were　obtained　by　solving　the　wave　equations　coupled　with　stress　and　electric　displacement　and　magnetic　field.　For　the　MEE　plate,　the　dispersive　curves　were　plotted　and　the　impact　on　the　Lamb-wave　phase　velocity　by　magnetic　field　was　discussed.　The　results　show　that　magnetic　field　excerts　obvious　effect　on　Lamb-wave　phase　velocity.