I am not good in Roman coins and need help.
Does any one know what coin is this?
It is a late Roman bronze, often called a centenionalis due to being tariffed at 100 denarii (yeah, inflation happened in ancient Rome, too). The reverse type is FEL TEMP REPARATIO, Roman soldier spearing fallen Persian horseman. Mintmark SMTS (Thessalonica).
The obverse type is trickier, as much of the emperor's name is missing or illegible and this type was struck under several Late Roman emperors. I think I can read DN CONST..., which makes it most likely to be Constantius II.