Usability has become a primary factor in determining the acceptability and consequent success of consumer product. It is common that the product usability is evaluated by objective performance measures and/or subjective user preference measures. This study is concerned with objective evaluation of the product usability using 3-dimensional motion analysis. We evaluated car-audio position using 3-dimensional motion analysis. The parameters investigated in this experiment were height of car-audio, left-right angle, and front-end angle. The experimental results showed that the usability evaluation method using motion analysis was consistent with user's subjective assessment. This objective method can be applied to not only car-audio position evaluation but also various consumer products'usability evaluation.