A data transmission method, a terminal, and a network device are provided, to improve reliability of data transmission performed by a terminal in a grant-free mode. In embodiments of the present invention, a terminal sends, in a grant-free mode, first data to a network device by using a grant-free resource, and if the terminal needs to switch from the grant-free mode to a grant mode, the terminal sends, in the grant mode, second data to the network device by using a grant resource. When the terminal initially transmits the first data in the grant-free mode, if the terminal needs to switch from the grant-free mode to the grant mode, the terminal may fall back to the grant mode and transmit data in the grant mode. In this way, reliability of data transmission is improved and a success rate of data transmission is improved to a relatively great extent.