The mental model is the root of usabilities of user interfaces. This article, by introducing the methods of Cognitive Psychology and Usabitlity Engineering, makes experiments to explore whether users could encounter difficulties in the mobile phone?s multimedia interface when the mental model formed in graphic user interface (GUI) is transferred into multimedia user interface (MUI). Experimental results show that 1) when the icon?s medium attributes change from the stactic state in GUI to the animation in MUI, users can?t generate conflicts in their minds, and 2) when information?s layout on the MUI?s 2nd level interface changes from the column usually used in GUI into the lattices consistant with the 1st level interface of MUI, the represention progress of user?s mental models will be quicken. Based on these, we think that changing information?s layout and adjusting metaphors of interface can improve the usability of MUI.