The utility model discloses a chain which comprises a group of series-connection units, wherein each series connection unit comprises a chain roll, inner chain sheets arranged at the two sides of the chain roll, outer chain sheets arranged at the outer sides of the inner chain sheets and pin shafts for connecting the chain roll, the inner chain sheets and the outer chain sheets; each outer chain sheet comprises a first connecting part, a second connecting part and a waist part; the first connecting part and the second connecting part are arranged at the two ends respectively; the waist part playing the role of connection is arranged between the first connecting part and the second connecting part; the waist part is provided with an upper chamfer and a lower chamfer; and the first connecting part and the second connecting part are both in an oval shape and form a consistent arc structure together with the upper chamfer and the lower chamfer. Therefore, the chain wheel and the outer chain sheets can be more tightly meshed in the speed change process, the noises are reduced while the speed changing is more smoothly carried out; and the lengths of the chamfers at the two sides are different, so that the purpose of more precise speed changing can be achieved; and meanwhile, the chain is more attractive and ornamental to meet the individualized requirements of customers.