Thanks for reply
But these are better than mining using expensive graphic card which are power hungry as well ? Right ?
are there any good usb miner for start mining...
They're certainly a good step up from a graphics card, however if you actually want to be making any profit you'll want to look at getting a few of something like the Antminer S9 which goes at about 14 TH/s each (14000 GH/s or 1750x faster than that wimpy USB miner).
You really need to be doing a lot of research before you invest a lot of money in something. For example 8GH/s at the current difficulty could be giving you around 0.00013288
BTC per month which probably only just covers the electricity bill. You could also try mining on your own, but with 8GH/s at the current difficulty you should be expecting to find a block every 7844 years.