According to historical records, surname Zhang derives from surname Jiang.
Jiang Ziya is one descendant of Emperor Yan that made great contribution to the founding of Zhou Dynasty. One branch of Jiang's descendants was conferred the place of Zhang and later they established State Zhang.
Later State Zhang was conquered by State Qi and the royal offspring of State Zhang adopted the left component of the character Zhang, the name of their perished state which is also pronounced Zhang, as their surname ever since.
Other historical materials recorded that Huangdi (Yellow Emperor) had once granted 12 surnames to his 12 outstanding sons, and among them there was one who took the surname Ren. Later the Ren surname flourished and split into ten sub-branches. One of them settled in the place of Zhang, and they adopted Zhang as name and multiplied and lived to this day.
By People's Daily Online