A data transmitting method suitable to a client and a server is provided. The method includes the following steps. The client generates an initial value. The server generates a first time in response to the initial value. The client generates a second time and a first verification hash code related with the first time and the second time. The server generates a random variable and a second verification hash code related with the first time, the random variable and the first verification hash code. The client generates a first transmitting data and a third verification hash code related with the first transmitting data, the first verification hash code and the second verification hash code. The server generates a first response message and a fourth verification hash code related with the first response message, the second verification hash code and the third verification hash code.