First of all overwrite the file

app/code/core/Mage/Catalog/Model/Layer.php

and copy this file to

app/code/local/Mage/Catalog/Model/Layer.php

In function getProductCollection(), Put this code after line #102

<?php
$collection->joinField(‘inventory_in_stock’, ‘cataloginventory_stock_item’, ‘is_in_stock’, ‘product_id=entity_id’,’is_in_stock>=0′, ‘left’)->setOrder(‘inventory_in_stock’, ‘desc’);
?>