Method and apparatus for transporting client signals in an OTN are illustrated. In one embodiment, the method includes: mapping a client signal into a first Optical Channel Data Tributary Unit (ODTU) frame including an ODTU payload area and an ODTU overhead area, such that a plurality of n-bit data units of the client signal are inserted into the ODTU payload area and number information is inserted into the ODTU overhead area; mapping the first ODTU frame into the OPUk frame, such that the plurality of n-bit data units are mapped into an OPUk payload part occupying at least one Tributary Slot (TS) of the OPUk payload area and the number information of the ODTU overhead area is mapped into a first OPUk overhead part of the OPUk frame; forming an Optical Channel Transport Unit-k (OTUk) frame including the OPUk frame for transmission.