I want to buy iphone in hongkong, is it unlocked?

iPhones do not have international warranty. It can be used in other country if it is not carrier locked.

The iPhone warranty has never been international and always includes the provision that service may be restricted to the original country of purchase. As to whether the particular device you buy is unlocked, you will need to specify and verify that fact when you buy it from wherever you end up buying it. Just be sure you get a phone that is carrier unlocked, not merely SIM-free (which often merely means contract free but not necessarily carrier unlocked.

I would also point out that the online Apple iPhone store is currently offline and being updated, as new product announcements are expected today. So you may wish to wait on any purchase, at least for today.