if Drg. specify Fillet size as 8mm, whether we have to check 8 mm as Throat or Leg ?

troat

It normally refers to the fillet leg length, here is a web site to help with what the symbols on the drawing show;

http://www.arcraftplasma.com/weldingsymbols.htm

Standard format is to dimension the leg of the fillet, the throat of a fillet is .707 of the leg size.

You might want to checkout this website.  I personally used this equipment.  It will eliminate any question.

http://www.qtm.net/~galgage/product.html

Just looked at a large fabrication with a tremendous number of cracks emanating from bad, bad filet welds.  All the welds in question had a very convex throat to the point that there was lack of fusion on  one or in some cases both toes of the weld.  

Here is an example of using the throat size gage when measuring fillet welds.

http://www.weldinginspectionsvcs.com/FilletWeldgage.htm

Gerald Austin
Iuka, Mississippi
http://www.weldinginspectionsvcs.com

GenB,
     If you think 8mm fillet refers to the throat (troat) dimension then suggest you are in the wrong profession.

Isn't the throat dimension used for strength calculations ?

Gerald Austin
Iuka, Mississippi
http://www.weldinginspectionsvcs.com

Yes, the throat dimension is used for strength calc's. But the weld is called out by its leg size. The throat dimension is something which is impossible to directly measure in the field; the leg dimension can be measured with inexpensive tools such as the fillet weld gage that pipewelder1999 provided a link to. Same concept as not calling out a "mean diameter" of a vessel measuring from the middle of the shell thickness. While it may make the calc's slightly more accurate, the shop has no reasonable way to take direct measurements from the middle of the shell thickness.

jt

In my opinion, it all depends on the detail/symbol used on the drawing. If using standard welding symbols such as that referenced in the link by metengr, then the leg shall be measured. If the drawing for some reason indicates other than a welding symbol, measure the dimension as shown on the detail. It may show throat dimension or a leg dimension. Use of proper terminology in these questions can eliminate all confusion.