how to apply jqury class in asp.net textbox if we use multiline mode.?

@jackparsana: A multiline ASP.NET TextBox is rendered as a <textarea> on the client, so jQuery selectors that target input[type=text] (or the jQuery :text pseudo) will not match it. Also remember ASP.NET naming containers can change the rendered id, so selecting by a literal id can fail unless you use the control's ClientID or set ClientIDMode="Static".

A couple of practical fixes:

$(function(){
  // select by the server control's client id
  $("#<%= TextBox1.ClientID %>").addClass("myJqClass");
});

$(function(){
  // select both text inputs and textareas
  $("input[type=text], textarea").addClass("myJqClass");
});

You can also set the server-side CssClass="myJqClass" on the TextBox so the class is emitted at render time.

If your page uses UpdatePanel/partial postbacks, classes added with jQuery may be lost after an async update. Reapply them on the endRequest event:

Sys.WebForms.PageRequestManager.getInstance().add_endRequest(function(){
  $("#<%= TextBox1.ClientID %>").addClass("myJqClass");
});

Notes and cautions: CSS rules that target only input elements will not style a textarea, so adjust selectors if you rely on CSS. The jQuery :text selector is for input[type=text] only; see the jQuery docs for details. For how ASP.NET renders the TextBox control and ClientID behavior, see the TextBox documentation.

jQuery :text selector
TextBox control (ASP.NET) documentation