Assuming they are 6 sides dice, the answer is 5/324.
The first die rolled has a 6/6 chance of being unique. The second die has a 5/6 chance because it cannot equal the value of the first die. The third die has a 4/6 chance, etc.
(5/6)(4/6)(3/6)(2/6)(1/6) = 120/7776 which reduces to 5/324.
