I am to design a street light using LED...can anyone please give me an idea on how can i create an LED light equal to the efficiency of the existing???
i mean it's brightness....and what kind of LED??/thank you so much..

 

Even the highest power LED's (5 watt) can't even start to compete with the LUX output of a street light.... be it halogen, tungsten, flouro, mercury vapour or Sodium.
A LOT of LED's would be required. I suggest you find the Light output specs of the LEDs you intend to use, and the specs of the street lamp then work it out for yourself.
BTW candle power is a function of beam width and Lumens, so if your LEDs are measured in mcd, that will complicate your calculations.
As a rough guide however, your average High power LED should produce around 25 lumens per watt.

OK, a quick "google" shows me that the average street lamp has an output of 20,000 to 30,000 Lumens.
If you want an efficient street lamp, stick with a 250 watt sodium.