My cat is being terrorised?

By another cat, me and my cat have recently moved to a appartment, at first my cat kept running back to the old area we lived, I had to keep brining him back. Now at last he seems to have settled but a slightly larger cat that lives in the appartment is constantly terrorising my cat what can I do to put a stop to it any ideas guys. She is forcing my cat to keep running away, this continues and he could get knocked down by a car as my old house is quite a distance.