I applied for L1B extension. Now I have got L1B Pettion as approved for 3 yrs and new I-94. But I can see my status in USCIS as 'Post Decision Activity'.
Is my extension process completed? or Something else is there?
No need to worry about the designation on the site. "Post Decision Activity" can refer to many things and it is just USCIS' way of describing any administrative funcitons they may have to take care of after a decision. It could include things like sending out the actual approval (that you already have).
If you have the approval notice in hand, it would appear that the USCIS site is in error. That said, you might request that your employer contact the Service's customer service line to confirm that all is in order for you.