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I'm thinking about this subject

is "about" in the right position
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Nader75I'm thinking about this subject

is "about" in the right position
Yes, see:
http://query.nytimes.com/search/query?query=%22thinking+about%22+&srchst=nyt

"Of" is fine too, but less frequent:
http://query.nytimes.com/search/query?query=%22thinking+of+the%22&srchst=nyt
Comments  
Yes, you are right.

We use "of,about" after "think" and before the object of the verb.
Students: We have free audio pronunciation exercises.
do you mean we can use also "of "

I'm thinking of this subject
 Marius Hancu's reply was promoted to an answer.