The embodiments of the present invention disclose methods and apparatuses for mapping processing and de-mapping processing in an optical transport network. a Low Order Optical Channel Data Unit (LO ODU) signal is mapped into a payload area of an Optical Channel Data Tributary (ODTU) signal in units of M bytes. M is equal to the number of time slots of a High Order Optical Channel Payload Unit (HO OPU) that are to be occupied by the ODTU signal, and M is an integer larger than 1. Overhead information is encapsulated to an overhead area of the ODTU signal. Thereafter, the ODTU signal is multiplexed into the HO OPU. In this way, an efficient and universal mode for mapping the LO ODU to the HO OPU is provided.