Ya I got It...
Here is the solution
<!DOCTYPE html>
<html lang="en">
<head>
<title>Hello World!</title>
<script src="Cesium/Cesium.js"></script>
<style>
@import url(Cesium/Widgets/CesiumWidget/CesiumWidget.css);
#cesiumContainer {
position: absolute;
top: 0;
left: 0;
height: 100%;
width: 100%;
margin: 0;
overflow: hidden;
padding: 0;
font-family: sans-serif;
}
body {
padding: 0;
margin: 0;
overflow: hidden;
}
</style>
</head>
<body>
<div id="cesiumContainer"></div>
<script>
var widget = new Cesium.CesiumWidget('cesiumContainer');
var url='http://localhost:8080/geoserver/wms'; //Geoserver URL
var layers = widget.scene.globe.imageryLayers;
layers.removeAll();
layers.addImageryProvider(new Cesium.WebMapServiceImageryProvider({
url : url,
layers: 'GIS_Demo'// Here just give layer name
}));
Sandcastle.finishedLoading();
</script>
</body>
</html>