The diameter of non-thermal arc plasma is an important parameter to determine the current density of arc that determines temperature and density of the electrons. The images of gliding arc discharge driven by magnetism at atmosphere are captured by using CCD method. The distribution of shade of gray of the arc can be drawn from the CCD photographs. The boundary and diameter of the arc are determined according to the specific curve fitting. The diameter projecting in the direction of motion of the arc (front viewed diameter) and the diameter projecting in the vertical direction of motion of the arc (side viewed diameter) are obttained. The relationships between the diameter and the arc current as well as the magnetic field are analyzed. The front viewed diameter and side viewed diameter are compared too.