Abstract: In the Jingdong dialect of Yunnan Province (is located in Southwest China), the word bang is frequently used as a preposition. It serves two functions: as a benefactive marker and a dispositive one in the dialect. Comparative analysis with other dialects reveals that the two prepositional usages of bang are not unique to Jingdong but exist in other dialects as well. From a diachronic perspective, the grammaticalization process of bang can be outlined as: a verb meaning to help> a benefactive marker> a dispositive marker.