A method and apparatus for data transmission are provided, which can accurately determine a transport block size (TBS) for a data channel in sidelink (SL) transmission, thereby improving transmission performance. The method includes the following. A terminal device determines number of resource elements (REs) for a physical sidelink shared channel (PSSCH) in a second resource according to a first resource used for transmitting a physical sidelink control channel (PSCCH) and the second resource used for transmitting the PSSCH, where the first resource at least partially overlaps with the second resource. The terminal device determines a TBS for the PSSCH according to the number of the REs for the PSSCH.