Travel by Car: The driving distance between Mostar and Belgrade on recommended route E 73 is about 430 km, which equals a travel time of about 6 hours. There is also an alternative route (M20) but the conditions of the road are not so good and travelling time is more than 7 hours.
Travel by Bus: Timetable, if any, for direct buses between Mostar and Belgrade you can see here, if there are no direct buses available it is possible to from Mostar to Sarajevo where you then switch to a bus to Belgrade, timetable and prices for buses from Sarajevo to Belgrade, you can check here.
Alternative ways of travelling from Mostar to Belgrade:
Travel by train: From Mostar there is train to Sarajevo. The trains route between Sarajevo and Belgrade have been a bit on and off the latest years, the current status of you can check here.
Travel by plane: On this route there is no direct flight.
Map with buses from Mostar: