javascript function calling inside a page_load event of asp.net

JavaScript is client-side code.

Page_Load is server-side code.

You can't call one directly from the other. The next best thing is to arrange for your client-side code to be called when the page loads in the browser. You can do that by dynamically generating your script in-line with the rest of your markup.

You can use Page.RegisterStartUpScript

  public void Page_Load(Object sender, EventArgs e)
  {

    if (!this.IsStartupScriptRegistered("Startup"))
    {
      // Form the script to be registered at client side.
      String scriptString = "<script language=\"JavaScript\"> function DoClick() {";
      scriptString += "showMessage2.innerHTML='<h4>Welcome to Microsoft .NET!</h4>'}";
      scriptString += "function Page_Load(){ showMessage1.innerHTML=";
      scriptString += "'<h4>RegisterStartupScript Example</h4>'}<";
      scriptString += "/";
      scriptString += "script>";
      this.RegisterStartupScript("Startup", scriptString);
    }