Hi prawinrajkuma...,

This is browser inbuilt functionality and it is different in each brower. You can't change it.

If you wan to display center of page use jquery or bootstrap modal popup to display the message.

Refer below articles.

Open (Show) Bootstrap Modal Popup Window from Server Side (Code Behind) in ASP.Net

Open ( Show ) jQuery UI Dialog Popup Window from Server Side ( Code Behind ) in ASP.Net

Thank sir, is there any other way to show alert popup in center of the page without using jquery & bootstrap.

or  we can make this popup customize to center.

protected void Page_Load(object sender, EventArgs e)
{
    string message = "Hello! ";
    System.Text.StringBuilder sb = new System.Text.StringBuilder();
    sb.Append("<script type = 'text/javascript'>");
    sb.Append("window.onload=function(){");
    sb.Append("alert('");
    sb.Append(message);
    sb.Append("')};");
    sb.Append("</script>");
    ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());

No, its not possible.