In the conventional UNIX system, the all device driver realized in the kern el. Hence, whenever we want to add a new device driver or change the existin g device driver, the modification of kernel is unavoidable. Generally, it is very difficult to modify the kernel codes.As a method of overcoming this difficulty, a TTY device driver model of user-level is presented in this paper. The basic concept of this model is providing a dynamic reconfiguration of TTY device driver by realizing a user-level server process for TTY device driver. In order to verify the proper y of this model, a prototype of TTY device driver has been realized in the SunOS and Linux environment and evaluated its performance.