A USB peripheral device and a method of determining USB speed mode. The method of determining USB speed mode for an USB peripheral device, wherein the USB peripheral device is coupled to an USB host through an USB transmission cable, and the USB transmission cable comprises power and ground lines for power supply, and first and second data signals for transmitting data, the method comprising the USB peripheral device pulling up voltage level of the first data signal with a first pull-up device, the USB peripheral device pulling up voltage level of the second data signal with a second pull-up device, and the USB host determining a high/full speed mode for the USB peripheral device, when detecting only one of the first and second data signals exceeding a threshold, and then detecting the other data signal exceeding the threshold in a predetermined period.