Just now I found that it's possible to provide feed for a specified
label of blog following Blogger's Help at

http://help.blogger.com/bin/answer.py?answer=53336&topic=9083

The format is:

http://blogname.blogspot.com/feeds/posts/default/-/labelname
or
http://beta.blogger.com/feeds/blogID/posts/default/-/labelname

However, when the label name contains a blank, for example, 'Knowledge
Base', the feed would be unavailable. Meanwhile, all single word label
and even labels in url-encoded Chinese Characters are all right
following this feed link.

I'm publishing via FTP and the HTML file of label 'Knowledge Base'  is
named 'Knowledge Base.html' and this would be automatically url-encoded
when access via URL as .../labels/Knowledge%20Base.html. If I put
'Knowledge%20Base' as the label name in the feed URL, it would lead to
a error: We're sorry, but we were unable to complete your request.

Any Suggestions?