I am using tinymce, Is it possible to apply for only one textarea

For the textarea assign a class="" to textarea property, this will support for you

<script type="text/javascript">
    tinyMCE.init({
        //mode : "textareas",
        mode : "specific_textareas",
        editor_selector : "myTextEditor",
        theme : "simple"
    });
</script>

<textarea id="txtdesc" name="txtdesc" class="myTextEditor" rows="6" cols="96" ></textarea>