The objective of the study is to investigate the influence of aesthetic attributes on user interface design. Theories of aesthetics are first studied from which aesthetic attributes are defined as information sources. Representation of a product is viewed as a combination of aesthetic attributes, based on a hypothesis that each attribute has a certain influence on the performance in terms of user interface. An experiment was conducted to support the hypothesis. Various combinations of aesthetic attributes of a microwave oven were generated from the orthogonal array of a conjoint analysis. Subjects perfomed on a computer two types of tasks, each of which simulated a combination of attributes. Reaction time was evaluated as a performance measure for the conjoint analysis to find out how much influence each attribute has on the performance. A computer program, UISA(User interface Simulation system of Aesthetic attributes0, was developed and used for simulation. Findings on the influences of aesthetic attributes were summarized.