<?php
  if (is_category()) {
    $cat_obj = $wp_query->get_queried_object();
    $kat_id = $cat_obj->parent;
    if ($kat_id == 0) {  //Eğer ana kategori ise
       $kat_id = $cat_obj->cat_ID; 

       echo '<ul>';
       wp_list_categories('child_of=' . $kat_id);
       echo '</ul>';

    }
  }  //is_category
?>