1st way:-
document.getElementById("txt_description").readonly = true;
2nd way:-
document.forms[0].txt_description.setAttribute('readonly','readonly');
3rd way:-
document.forms['form1'].txt_description.setAttribute('readonly','readonly');
4th way:-
document.forms['form1'].txt_description.readonly = true;
//txt_description is defined in aspx page as:
<asp:TextBox ID="txt_description" runat="server" TextMode="MultiLine"></asp:TextBox>