Frost　formation　on　a　horizontal　copper　surface　under　low　air　temperature　and　forced　convection　conditions　is　investigated　experimentally.　Both　the　frost　crystals　pattern　and　the　frost　layer　thickness　formed　on　the　cold　plate　are　compared　under　different　experimental　conditions.　The　environmental　variables　considered　in　this　study　include　the　ambient　temperature　(T-infinity),　air　relative　humidity　(phi),　and　velocity　(v),　as　well　as　the　cold　surface　temperature　(T-w).　The　tested　ranges　are　-5　＜=　T-infinity　＜=　5　degrees　C,　50%＜=phi　＜=　80%,　2.2　＜=　v　＜=　8.0　m/s,　-16.8　＜=　T-w　＜=-25.5　degrees　C.　The　experimental　results　show　the　cold　surface　temperature　and　the　air　relative　humidity　have　obvious　effects　on　the　frost　growth:　the　frost　layer　thickness　increases　strongly　with　the　decreasing　cold　surface　temperature　and　increasing　air　relative　humidity.　The　air　temperature　and　air　velocity　or　Reynolds　number　are　also　important　factors　affecting　the　frost　crystals＇　growth　and　thickness.　With　the　increase　of　the　air　temperature　and　velocity,　the　frost　crystals　become　denser,　and　the　frost　layer　thickness　become　thicker,　but　this　trend　becomes　weaker　under　higher　air　temperature　and　velocity.