For the same reason "table" doesn't work as a name in English: because it sounds weird to native speakers.

Also, teeechnically since you're not Japanese but have a Japanese name, you are supposed to write your name in katakana, not kanji.

i just can't believe that this thread was entertained for so long o.O'
like the guy said above, you can't have a name in kanji.
in katakana, yes, but you prolly wouldn't be too satisfied w/ the way it's pronounced nor written
and.... why, for the love of satan, did you decided to "change" your name to yuki? o.O'

im' sorry... but that's just silly >.>''

So is it your nick name? or did you legaly change your name to Yuki.
Also,
....why did you put san at the end of your sign in name

So you decided to change your name. You do it and change it into a foreign language you don't know and pick a random word whose meaning you ignore.

That beats by far people coming in here asking for a tattoo word. As per how to write it, what does it matter? You are not Japanese nor speak it. I didn't understand if you live in Japan (but I doubt it), so spell it the way you like and pass it for Yuki.
(And why did you use Yukiko as a screen name?)

Yuuki and Yuki do not sound the same. To Japanese ears they might as well be the difference between Steven and Charlie.

But your second point is the more important one.

Considering he/she is not Japanese I think it is probably less OK. Since when do people all of a sudden get to give themselves names in kanji?

What "chance" is this?

The only chance I can think of is if you are changing to a Japanese citizenship. Is that what is happening?

Put it this way. You are not Japanese. You are not from Japan. Your name is not Yuki, Yuuki or Yukiko, so it really doesn't matter what kanji you choose to write it, as it isn't your name anyway.

Alright, once and for all, this is the definitive answer to OP's question(s):
How do you write "Yuki" in Japanese? ユキ
How do you write "Yuuki" in Japanese? ユーキ

I know, just taking that little short cut was just the easiest way to make the connection between the difference in stephen and steven. ^^; No offense to the OP, but considering she asked why it doesn't work, I don't think she is entirely concerned with the difference between yuuki and yuki.

well, I understand how different yuuki and yuki can sound.
I'll change my choice then..

And just WOW!
So many questions about my freaking name!!
yuujirou : it is SOOOO not of your business why I changed my name! :eek:
I don't have to discuss that part of my life with all of you!

chryuop : yukikosan is a screenname right?! a SCREENname... I could have chosen IAMME187238 if I wanted to!
What's the problem with my screenname?! can't I chose the screenname I want when I register to a forum or must I ask permission to the members before!?

Son of a gun I NEVER thought I would see THAT MUST judgement from all of you!! :eek:

I came here to have some answers, and I got them!
Thanks but dudes, you really aren't nice to other people!!
Just because you don't understand why I changed my name, LEGALLY, that doesn't give you the right to judge me.

Yes, I didn't know what Yuki meant when I picked it.. BIG DEAL!!!
I didn't know what Nadjat meant either when I gave this name to my little girl!! SO WHAT!
Is that really an issue or what!...

Yeah, generally I know that it is true, I was simply thinking about foreigners who become Japanese citizens. I believe they have to choose Kanji for their names.