The Yuan Dynasty Urn is the twenty-seventh treasure that can be found in Uncharted 2: Among Thieves.
The Yuan Dynasty Urn is found in Chapter 6 - Desperate Times . This is a fairly well-hidden treasure, though acquiring it shouldn't be all that hard. Going through this chapter you will recognize a stairwell with an elevator shaft between. The idea is to head up to the very top of this stairwell. The stairs will eventually break off, and it'll appear as if you'll be unable to continue. However, this couldn't be farther from the truth. Jump from the break in the stairs towards the extra small elevator shaft running vertically down the middle of the stairs. When you're hanging from the top of it, all you have to do is pull yourself over the cage so that you land atop the elevator car itself. This is where the treasure can be found.