hello,
I bought a arduino UNO R3 and i want to use it 24*7 for a year atleast.So can i use it
Yes. It has no moving parts. It will run as long as you provide power and don't hit it with anything.
Take some steps to ensure your connectors can't vibrate out.
Thanks i will us it 24*7 with damped vibrations and will provide a fan to keep it cool 
The Arduino won't run hot unless you power it from 12V, and the regulator is what gets warm.
If you can power it from 7.5V, it won't be hot at all.
A fan blows dust onto the Arduino board. No fan please. Use the voltages wisely as CrossRoads wrote.
A year is peanuts. I think an official Arduino board can run at least 24/7 for 10 years.