Bitumen was modified in order to increase the performance and decrease the cost of bitumen or for both purposes. This paper presents the effect of molasses on performance of bitumen and asphalt concrete mixtures and determination of optimum molasses content in the asphalt mix with 3 types of NMAS (19mm, 12.5mm and 9.5mm). Marshal specimens were prepared using virgin 60/70 and 80/100 penetration grade bitumens to obtained optimum binder content (OBC) for each grade of bitumen. The remaining marshal specimens were prepared by adding 3%, 6%, 9%, and 12% of molasses by volume of OBC in each type of NMAS. The replacement was done by subtracting equal volume of bitumen from OBC. Bitumen quality tests of virgin bitumen and modified bitumen also conducted. The modified bitumen marshal test result and bitumen quality tests were determined and evaluated according to Ethiopian Road Authority (ERA) and American Association of State Highway and Transportation Officials (AASHTO) specifications. Based on the result 6% molasses was the optimum molasses content used to modified 60/70 penetration grade bitumen and 10% ,10% and 9% was the optimum molasses content for 80/100 penetration grade of bitumen with 19mm,12.5mm and9.5mm NMAS respectively. Other volumetric properties were within the limits of the ERA and AASHTO specifications. The resistance of mixes against moisture was also evaluated according to AASHTO T 283 moisture sensitivity test and the mixes showed decrease in moisture resistance as percentage molasses increased in the bitumen.