Dear all, Any one can explain me how to plot derating curve for a 25VA transformer @ ambient 60° Temperature.

Pls advice

 

You need to know the specified maximum operating temperature rating of the transformer to calculate the derating for higher temperatures.
Since transformer heating is primarily due to I^2R losses in the winding resistance you would derate the current (VA) on a square-law basis.

 

Maybe more important is the type of load. Have a look here.
http://www.hammondmfg.com/pdf/5c007.pdf