How we provide the source dynamically for Iframe tag ?

Try this.
Write this code on ASPX

 <div runat="server" id="Iframe">  
 </div>

Write this code on ASPX.CS

using System.Text; // Namespace

        string URL = "http://www.dotnetfunda.com"; // Set your custom url.
        StringBuilder sb = new StringBuilder();
        sb.Append("<frameset cols='35%,80%'> ");
        sb.Append("<iframe name='frame1' src='" + URL + "'> ");  
        sb.Append("</frameset> ");
        Iframe.InnerHtml = sb.ToString();