Dear Sir,
I am working in Saudi Arabia, Riyadh, and I am currently looking more than 100000sq meter lawn (Bermuda grass: Cynodon dactylon). My problem is the weed, Poa annua commonly called blue grass. Can any one suggest me for some specific week killer which kill only the weeds not the Bermuda grass.

Try this site for some tips:

CONTROL of POA (http://www.agry.purdue.edu/turf/pubs/agry9802.htm)

A quote from the site:

Control
There is no selective control of Poa trivialis today, though scientists are trying to develop one. Nonselective control with glyphosate or glufosinate followed by reseeding may offer the best chance of control of Poa trivialis.