thanks for your help ...
I have change code as you say above like this

....
switch($order_status_id){
case 1:
// i change number 1 with order_status ID table in phpmyadmin.
if (file_exists(DIR_TEMPLATE . $this->config->get('config_template') . '/template/mail/pending.tpl')) {
$html = $template->fetch($this->config->get('config_template') . '/template/mail/pending.tpl');
} else {
$html = $template->fetch('default/template/mail/pending.tpl');
}
break;
case 2:
// i change number 2 with order_status ID table in phpmyadmin.
if (file_exists(DIR_TEMPLATE . $this->config->get('config_template') . '/template/mail/processing.tpl')) {
$html = $template->fetch($this->config->get('config_template') . '/template/mail/processing.tpl');
} else {
$html = $template->fetch('default/template/mail/processing.tpl');
}
break;
case 5:
// i change number 5 with order_status ID table in phpmyadmin.
if (file_exists(DIR_TEMPLATE . $this->config->get('config_template') . '/template/mail/complete.tpl')) {
$html = $template->fetch($this->config->get('config_template') . '/template/mail/complete.tpl');
} else {
$html = $template->fetch('default/template/mail/complete.tpl');
}
break;
//this executes if all other cases fail.
default:
if (file_exists(DIR_TEMPLATE . $this->config->get('config_template') . '/template/mail/order.tpl')) {
$html = $template->fetch($this->config->get('config_template') . '/template/mail/order.tpl');
} else {
$html = $template->fetch('default/template/mail/order.tpl');
}
}
and admin area ... i go to sales - order - order history and change status order pending to process .. there are no send custom email processing.tpl .... I probably made a mistake xD ...
still send email normal ...
"Order ID: 36
Date Ordered: 13/05/2012
Your order has been updated to the following status:
Processed
To view your order click on the link below:"