This application provides a data transmission method and apparatus. The method includes: processing, by a network device, a to-be-sent optical data unit ODU to obtain another ODU, where a bit rate of the another ODU is lower than a bit rate of the ODU; and sending, by the network device, the another ODU. In embodiments of this application, the ODU is processed to obtain the another ODU with a lower bit rate, and this helps reduce a rate increase when service data is transmitted in an OTN, so as to reduce an OTN interface rate and OTN costs.