Hi, Arriving in Hong Kong Aiport at 12 midnight. How to get to Mongkong Station by Bus?

Do you refer to HK Station?
If yes, upon landing, take bus no.N11 at Passenger Terminal Building, Cheong Tat Road and go to Li Yuen Street East, Des Voeux Road Central. The first schedule is at around 00:50. After that, walk northeast around 280 yards. The duration is about 75 mins. It takes 31 HKD per person.
In addition, the last schedule of Airport Express is at 00:48. If you can catch it, just take it to HK Station directly.

Or do you refer to Mong Kok Station?
If yes, take bus no.N21 at Passenger Terminal Building, Cheong Tat Road, and go to Argyle Centre, Nathan Road. Then walk south along Nathan Road around 120 yards. It takes around 75 mins. The fare is 23 HKD per person.