The invention provides an error detection method for data transmission in a transmission system with a first device, a second device and a data line. The method comprises the following steps. Firstly, a clock signal is sent to synchronize the first and second devices. Secondly, at least one serial data is sent from the first device to the second device in response to the clock signal. After sending the serial data, the first device sends an acknowledgement signal to the second device. After sending the acknowledgement signal, the first device reads the data line to obtain a reading value and determines whether the serial data is correct according to the reading value and a default value of the data line.