Hello all i want to mesuare 1 mg to 100 mg weight using arduino but i confuse about sensor
i.e load cell is accurate for such small mesurment or any other sensor is avaible for such small weight mesurment 
What is your budget?
Expect to pay about $1300 for a scale with 1mg resolution and an RS232 interface. See http://scientech.balances.com/
If you want to build something than perhaps measure the deflection of a thin glass fibre using a mirror and laser.
You might be able to hack something like below.