I found it!!!! on tax form 8606 instruction page 3 it says as below
Reporting recharacterizations. Any recharacterized conversion or Roth IRA rollover from a qualified retirement plan will be treated as though the conversion or rollover had not occurred.
I am going to reply IRS and quote this part of sentences. my guess is, IRS thought I own tax because I did full conversion in 2010 and full recharacterization in 2011. although the two transactions were for the same tax year, they fell into 2 calender years. IRS didn't know or notice that I did full recharacterization in april 2011. they thought I have conversion for 2010 so they request tax for it.
