Where to know the list of NLTK tagset?
No one answered the question, then I will answer it myself. Thanks to @user12075 for the link. I didn't find it when I was googling it. 

https://stackoverflow.com/questions/15388831/what-are-all-possible-pos-tags-of-nltk

From the above link, I know that nltk uses The Penn Treebank's POS tags.

nltk.help.upenn_tagset() will give you the list.