A positioning structure of clamp may include a fixed clamp and a guiding rod, and the fixed clamp has a clamping portion and a connecting portion respectively at an upper portion and a lower portion thereof, and a guiding hole penetrates through the connecting portion. The guiding rod coupled into the guiding hole comprises a plurality of first through holes thereon, a first engaging hole and a second engaging hole respectively penetrating through two lateral sides of the connecting portion are respectively communicated with the guiding hole. A plurality of lock holes are formed at the lateral side of the connecting portion same as the second engaging hole. A positioning unit, connected to the connecting portion through the first engaging hole, the second engaging hole, and the lock holes, comprises a clamping block, an engaging block, a positioning bolt, a positioning block, a clamping plate and a rotating member.