Abstract: The Mach I/O system works with conceptions and structures different from UNIX.Based on two Mach conceptions（the port and memory object）.the device management provides flexible RPC user interface. In the new version of Mach 3.0, the I/O system be implemented by user servers.This paper explains the method of Mach device management, "device-independent" driver, direct control of device in user space, and the performance.