Thanks.
I changed mail/html/order_invoice.tpl ,
Code:
<b>{$lng.lbl_po_number}:</b> {$order.po_number}<br />
to
Code:
{if $order.payment_method eq "Purchase Order"}
<b>{$lng.lbl_po_number}:</b> {$order.po_number}<br />
{/if}
But this does not display PO Number field even if the payment method is Purchase Order. I don't know what is wrong.