Goto app/code/core/Mage/Catalog/Model/Resource/Product/Collection.php
copy to app/code/local/Mage/Catalog/Model/Resource/Product/Collection.php
find function addCategoryFilter(
and here you you find code
public function addCategoryFilter(Mage_Catalog_Model_Category $category)
{
$this->_productLimitationFilters['category_id'] = $category->getId();
/* start to comment here
if ($category->getIsAnchor()) {
unset($this->_productLimitationFilters['category_is_anchor']);
} else {
$this->_productLimitationFilters['category_is_anchor'] = 1;
}*/
/* new line */
$this->_productLimitationFilters['category_is_anchor'] = 1;
if ($this->getStoreId() == Mage_Catalog_Model_Abstract::DEFAULT_STORE_ID) {
$this->_applyZeroStoreProductLimitations();
} else {
$this->_applyProductLimitations();
}
return $this;
}