The relation between the radial potential profile of the core plasma are examined in the tandem mirror GAMMA 10. It is observed that the radial potential profiles are convex over the cross section of the plasma column and the high energy end-loss ions over the confinement potential are decreased too in many shots. The radial potential profiles were controlled by changing the electrostatic potential profiles of the separated coaxial end plates. On flattening the radial profiles, the reduction of the high energy end-loss ions was softened and the plasma density increased in the central cell. These results suggest that the flattened radial potential profiles decrease the radial diffusion and improve the confinement of the plasma.