USB (Universal Serial Bus) is a far widely used bus. It has many advantages, such as plug and play, hot plug and small volume of the interface, which have hugely changed the development of the technology of connecting computers to external equipments．μCOS-II is an open-source, complete, portable, ROMable, well custom real-time embedded operating system, which can be easily ported to a variety of microprocessor. The paper uses the LPC2378 reader card which I had used in the SUNTEK as an example to introduce the design of USB host system in μCOS-II.