From Belgrade you can follow the A1 to Paracin, then along the 36 to Zajecar (see later) or drive along the Danube. For the Danube turn left off the A1 after Smederevo; follow Pozarevac to Veliko Gradiste. From here follow the river visiting Golubac Castle & Lepenski Vir. Continue along the river. You may turn right after Donji Milanvac, following the 165 & 33 to Stubik, then Negotin. You may also follow the river to Kladovo & from here drive south on the 35 to Negotin.From Negotin head towards Bregovo/ Vidin (BG) on the 33. As you near the border follow signs for Rajac which will take you left & in a bridge under the Vidin road. Stay on this road to Rajac.
All roads lead to Zajecar (well some do). From Zajecar the most interesting drive is to follow the border road passing through Halovo, Mali Jesanovac, Sipikovo & on to Rajac. The road is the 169 & has wonderful views. From Zajecar it is also possible to follow the 35 North to Negotin, turning right to follow the 398 through Brusnik; a slightly quicker route.
Follow the A1 heading North to Paracin, then along the 36 to Zajecar (See Later). If you wish you may drive the mountain road 35, followed by E-771, via Svrljig & Knjazevac then onto Zajecar; a more picturesque route.