Along with the progress of intelligent control and robot technology, machine arms, which are not so easy to be effected by the environment, easy operated, have taken place of human in more and more field. The motor used in the joint of machine arm is very important and has more influence on the system. The choice of the motor and relative method of control has more or less effect on the cost, operating precision and the reliable of the arm. Among all the motors that can be used in the joints, Brushless DC motor (BLDCM), which has advantages as low cost, light quality and high reliable etc., has taken more and more attentions. In this paper, we analyze the characters of one kind of the high-energy density designed BLDCM in robot according to environment and its structures.