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()) ?>
try this another code:-

     <?php
       productCollections=$_category->getProductCollection()->addAttributeToFilter('type_id','configurable');
     foreach($productCollections as $p)
    {
     echo $p->getSku();
     }
     ?>