A soft keyboard is popular for inputting texts on the display of smartphone. As it is a keyboard in display, it has an advantage that can be easily changed unlike the hardware keyboard. An adaptive soft keyboard is needed as different types of people use smartphone in various situations. In this paper, we propose a hybrid system that predicts user behavior patterns using smartphone sensor log data based on random forest and generates the appropriate GUI to the predicted behavior patterns by the rules constructed from users’ preference. The random forest for predicting user behavior patterns has a high generalization performance due to the ensemble of various decision trees. The GUI mapping rules are constructed according to the data collected from 210 users of different ages and genders. Experimental results with the real log data confirm that the proposed system effectively recognizes the situations and the user satisfaction is doubled compared to the conventional methods.