The embodiments of the present invention disclose method and apparatus for mapping and de-mapping in an optical transport network, where the mapping method includes: constructing an Optical Channel Data Tributary Unit (ODTU) according to an amount M of time slots of a High Order Optical Channel Payload Unit (HO OPU) to be occupied by a Low Order Optical Channel Data Unit (LO ODU); mapping the LO ODU to a payload area of the ODTU in a M-byte granularity; encapsulating overhead information to the payload area of the ODTU; and multiplexing the ODTU, which has been mapped the LO ODU and encapsulated with the overhead information, to the HO OPU, so as to provide a high-efficient and universal mode for mapping the LO ODU to the HO OPU.