This paper presents an experimental evaluation of a collaborative task for a design assessment of a wearable robot arm. Wearable robotic devices have been recently proposed as a new concept of the human robot collaboration. In particular, wearable robot arms have been expected to complement our physical capabilities and perform multiple tasks simultaneously. However, a design principle of a wearable robot arm based on the cooperativeness and collision safety has not been discussed sufficiently. The design factors of the wearable robot arm are considered to have a dominant influence on the cooperativeness and collision safety. We therefore conducted an experiment to evaluate time required for a collaborative task and number of collisions with a robot arm device. Arm lengths and attachment positions were compared in the conducted experiment as a design factor of a wearable robot arm. The experimental results indicated a correlation between evaluation indexes and design factors. As a result, we suggest that the concept design of a wearable robot arm is recommended to consist of an extended arm length without constraints of the attachment positions.