This application provides a method for determining a transport block size and a communication apparatus, which may be used in systems such as vehicle-to-everything, V2X, and V2V. A receive-end terminal device and a transmit-end terminal device may determine a quantity of encoded symbols based on a symbol length used for sidelink communication in a time unit and an scaling factor, and may further determine a transport block size of a data channel based on the quantity of encoded symbols. Therefore, for all slots used for sidelink communication, a same quantity of encoded symbols can be determined, and a same transport block size can be further determined.