A new type of wearable robot that assists the user with additional arms has been presented in recent years. Previous studies have reported that wearable robot arms allow wearer to perform multiple tasks simultaneously. Although concept designs have been qualitatively discussed, quantitative consideration in the design phase of wearable robot arms is necessary to improve their safety and operational efficiency. In order to evaluate the man-machine cooperativeness between users and wearable robot arms, the present paper proposes three evaluation indexes: workspace extensiveness, cooperativeness and invasiveness. The proposed evaluation indexes were defined according to the calculations of the common domain of the human workspace and the robot arm workspace. The calculation results suggest a design tradeoff between the work efficiency and the workspace invasiveness. The proposed indexes can be utilized as a new parameter for evaluating human-robot interactions of wearable robot arms.