Question: She is not fat.She is not thin.If I change this sentence into ''neither--nor'' using ''inversion style'',

> 
> 1.Neither thin nor fat she is.
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> 2.Neither is she thin nor is she fat.
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> 
> 

Which is more correct?

Answer: 
It is more usual to keep the normal word order for the first clause and use inverted order for the second clause of such sentences, in a *not-neither* or *not-nor* pattern:

> 
> She is **not** thin, **nor** is she fat.
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> 
> She is **not** thin; **neither** is she fat. 
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> 
> 

(Of course, the most usual phrasing would be "she is neither thin nor fat", but I've assumed you're interested in inverted constructions in particular.)

Of your original options, I think **2** is technically correct, but belongs to a poetic style. **1** is incorrect (it might be technically correct if you changed "she is" to "is she", but would sound a bit odd). I would advise against such constructions.

In informal speech, people might say something like **1** with the "she is" word order and an added pause. It would then be acceptable very informally, but it wouldn't be an example of inversion. It would be a colloquialism (perhaps representing the addition of an afterthought in disconnected or disfluent speech), along the lines of "A good man, he is" or "A sunny day, it is".
From the sentences you have provided, let me go out on a limb and point out that the first sentence is grammatically incorrect. The second sentence is correct, and clearly the best among the two. 

You could use any of these:

> 
> * Neither is she thin nor is she fat.
> * She is neither thin nor fat.
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> 
> 

Also, in my personal opinion, you could use '*fat*' before '*thin*'. For me, the sentences sound better/ natural that way. But then again, it's totally up to you.