I wrote the same code with just a few changes :

Instead of panel I used one of its subclasses : FormPanel or GridPanel :

var simple = new Ext.form.FormPanel({

frame:true,
title: 'Start Form',
items: [
{
xtype: 'label',
text : 'welcome to our Electronic Banking Site'
}

]
});

and it didn't work!
I tried to find out the reason! I follow it until the time the container's specific "layout" renders its components! But I couldn't understand the problem. So, why doesn't it work?