Experimental results on plasma instability and potential confinement configuration in MM-4U tandem mirror are presented. The measured axial potential profile shows that the axisymmetric negative plasma potential tandem mirror confinement configuration has been established; the obtained plasma density profile is similar to the potential profile. The relationships between the plasma parameters and the eng-neering parameters have also been studied prelimilarily. The low frequency instability was observed at low plasma density in the central cell. By means of the linear spectral analysis techniques, the instability mode has been determined. It is suggested that the instability is excited in the low density plasma when the electron beam drift velocities exceed the ion acoustic speed.