How to get sub-categories of a specific parent category?

<?php 
$parentCategoryId = 107;
$cat = Mage::getModel('catalog/category')->load($parentCategoryId);
$subcats = $cat->getChildren();

?>

// Get 1 Level sub category of Parent category.

<?php 

foreach(explode(',',$subcats) as $subCatid)
              {
                $_category = Mage::getModel('catalog/category')->load($subCatid);
  if($_category->getIsActive()) {
    echo '<ul><a href="'.$_category->getURL().'" title="View the products for the "'.$_category->getName().'" category">'.$_category->getName().'</a>';
   echo '</ul>';
  }
}

?>

// Get 2 Level sub category of Parent sub category

<?php 
foreach(explode(',',$subcats) as $subCatid)
{
  $_category = Mage::getModel('catalog/category')->load($subCatid);
  if($_category->getIsActive()) {
    echo '<ul><a href="'.$_category->getURL().'" title="View the products for the "'.$_category->getName().'" category">'.$_category->getName().'</a>';
    $sub_cat = Mage::getModel('catalog/category')->load($_category->getId());
    $sub_subcats = $sub_cat->getChildren();
    foreach(explode(',',$sub_subcats) as $sub_subCatid)
    {
          $_sub_category = Mage::getModel('catalog/category')->load($sub_subCatid);
          if($_sub_category->getIsActive()) {
              echo '<li class="sub_cat"><a href="'.$_sub_category->getURL().'" title="View the products for the "'.$_sub_category->getName().'" category">'.$_sub_category->getName().'</a></li>';

 }
     }
     echo '</ul>';
  }
}
?>

// Get 3 Level sub sub category of Parent sub sub category

<?php 
foreach(explode(',',$subcats) as $subCatid)
{
  $_category = Mage::getModel('catalog/category')->load($subCatid);
  if($_category->getIsActive()) {
    echo '<ul><a href="'.$_category->getURL().'" title="View the products for the "'.$_category->getName().'" category">'.$_category->getName().'</a>';
    $sub_cat = Mage::getModel('catalog/category')->load($_category->getId());
    $sub_subcats = $sub_cat->getChildren();
    foreach(explode(',',$sub_subcats) as $sub_subCatid)
    {
          $_sub_category = Mage::getModel('catalog/category')->load($sub_subCatid);
          if($_sub_category->getIsActive()) {
              echo '<li class="sub_cat"><a href="'.$_sub_category->getURL().'" title="View the products for the "'.$_sub_category->getName().'" category">'.$_sub_category->getName().'</a></li>';
              $sub_sub_cat = Mage::getModel('catalog/category')->load($sub_subCatid);
              $sub_sub_subcats = $sub_sub_cat->getChildren();
              foreach(explode(',',$sub_sub_subcats) as $sub_sub_subCatid)
              {
                $_sub_sub_category = Mage::getModel('catalog/category')->load($sub_sub_subCatid);
                if($_sub_sub_category->getIsActive()) {
                    echo '<li class="sub_cat"><a href="'.$_sub_sub_category->getURL().'" title="View the products for the "'.$_sub_sub_category->getName().'" category">'.$_sub_sub_category->getName().'</a></li>';
                }
              }
           }
     }
     echo '</ul>';
  }
}

?>



If you want 4th, 5th,.... or n level category and so on sub categories than simple add this script.
Note:- For n level category please use recursion


<?php 

 // $sub_subCatid this is sub category id . means if you want 4 level than it will take 3 level subcategory Id.

 $sub_sub_cat = Mage::getModel('catalog/category')->load($sub_subCatid);
              $sub_sub_subcats = $sub_sub_cat->getChildren();
              foreach(explode(',',$sub_sub_subcats) as $sub_sub_subCatid)
              {
                $_sub_sub_category = Mage::getModel('catalog/category')->load($sub_sub_subCatid);
                if($_sub_sub_category->getIsActive()) {
                    echo '<li class="sub_cat"><a href="'.$_sub_sub_category->getURL().'" title="View the products for the "'.$_sub_sub_category->getName().'" category">'.$_sub_sub_category->getName().'</a></li>';
                }
              }

?>