This paper presents an experimental evaluation of a usability of a wearable robot arm in different combinations of arm mechanics and attachment positions. A wearable robot arm has been recently proposed as a new system concept for the assistance of activities of daily living. Wearable robot arms are expected to enhance our physical abilities and perform multiple tasks simultaneously. However, the usability study of a wearable robot arm has not been discussed sufficiently in the concept design phase. In particular, an experimental comparison of arm mechanics and attachment positions has not been verified in the previous work. We therefore conducted a usability evaluation with a hypothetical attached robot arm which performs a representative task of activities of daily living. The time required for the experimental task and score of NASA Task Load index was evaluated. Two design concepts of a wearable robot arm is suggested according to the usability analyses: shoulder attached articulated robot arm and chest attached spherical robot arm.