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If it hasn't arrived by the 25th (Christmas Day), get in touch with Google and they will reissue the cheque.
Alternatively (depending on the amount involved) pay extra for secured express delivery (takes 5-7 days).
Unfortunately Korea is not on the list of countries that Google do direct bank transfers (sometimes called electronic funds transfer) which is below. If you do have a bank account and would prefer to be paid that way - let Google know - after a few years of nagging by various publishers they did provide it instead of cheques. Just out of interest is it North or South Korea you are referring to? :) :-
List of countries EFT is allowed to for Google Adsense publishers