Step 1. Navigate to the /app/design/frontend/default/your_custom_template/template/catalog/product/list.phtml or Navigate to the /app/design/frontend/base/default/template/catalog/product/list.phtml open the file in the editor of your choice.
Step 2. Add the code below in the appropriate position where you want to display the SKU .
<?php echo nl2br($_product->getSku()) ?>
<?php echo $this->htmlEscape($_product->getSku()) ?>
<?php echo $sku =Mage::getModel('catalog/product')->load($_product->getId())->getSku();?> -