Except for hardware design, the user interface of mobile devices creates the first impression for users. Mobile devices such as cellular phones are used to expose users' characters and preferences. Sounds, fonts, colors, and background images are used to reveal users' personalities. The demand for means of expressing one's character via mobile devices has been raised by advanced display technology, higher resolution and many more colors. Flash Lite of Adobe Systems, Inc and uiOne of Qualcomm, Inc reflect these requirements, and achieve good results. But Flash Lite and uiOne have weakness as other solutions do. Users cannot exceed the functionality that each user interface system provides. This paper introduces new downloadable user interface which overcomes the disadvantages of other solutions. The downloadable user interface includes multiple UI Engines which supports different types of user interface. The downloadable user interface has been deployed on the Linux based smartphone of ETRI.