where does the drupal6 cck field default value stored in database?
No need to go directly into the database for that, you can use the content_field_instance_read() function:

module_load_include('inc', 'content', 'includes/content.crud');
$definition = content_field_instance_read(array('field_name' =&gt; 'field_foo'));

// Your mileage may vary here, inspect $definition[0]['widget'] to see what you've got available.
$default = $definition[0]['widget']['default_value'][0]['value'];


If you're dead set on going into the database, the info is stored as a serialised PHP string in the widget_settings column of the content_node_field_instance table.