this 980kv motor gives 730grms of thrust with 9*6 apc prop,20 amps esc & 2200mah 3s 20c lipo,what will be the thrust if prop is replaced with 6*6 apc?

this is roughly calculated ......but should be around 350 gms on 12 v....

requesting senior members to correct me 

The chart shows 8220 rpm at 12v, ie, about 75% of no load rpm (980x12)
Assume 90% with smaller load of 6x6 prop instead of 9x6.
Or say 10-11000 rpm.
APC may not have a 6x6.
But data for an APC 6.5x6.5 is available on
http://flybrushless.com/prop/view/281
Which shows about 300gms thrust...

If i remember from your other thread, you are making an ATR type twin engined airliner of 1.5 kg est weight.
In case this is your first model, don't mind, but its not a good choice.
Secondly, if may be better to keep all your queries related to a single model, in a single thread.
For example, it is not clear why you are considering 6x6 props for motors rated for 9x6, 10x4.7 etc
Best wishes