How to get List of Sub Category by main Category ID in Magento

In Magento default the "top.phtml" file shows all category and sub category. can access all category and subcategory in magento by editing "top.phtml"

Here is the location for "top.phtml"
app/design/frontend/your-package/your-theme/template/catalog/navigation/top.phtml

For that we will write the code in top.phtml file

<?php foreach ($this->getStoreCategories() as $_category): ?>
<li>
<a href="<?php echo $this->getCategoryUrl($_category)   ?>"><?php echo $this->htmlEscape($_category->getName())   ?></a>
<?php 
$_catid=$_category->getId();
$category = Mage::getModel('catalog/category')->load($_catid);
$subcategory =  $category->getAllChildren(true);
array_shift($subcategory);
if($subcategory!=null)
{?>
<ul>
<?php
 
foreach ($subcategory as $sub)
{
$sub1 = Mage::getModel('catalog/category')->load( $sub);
?>
<li><a href="<?php echo $sub1->getUrl();?>"><span>
<?php echo $sub1->getName(); ?>
</span></a></li>
<?php } ?>
 
</ul>
<?php  }?>
 
</li>
<?php endforeach ?>