Cylindrical targets of 40Cr steel were immersed in a nitrogen plasma with density 1 x 10(10) cm(-3) and implanted at an energy of 30 keV. In order to characterize the inner surface modification effect and the dose uniformity achievable with a new plasma source ion implantation method, the axial distributions of the microhardness, and the implantation dose on the inner surface of the targets were measured, respectively. The experimental results demonstrate that the inner surface of materials can be effectively modified with our method and the measured root-mean-square variations of the retained dose and the microhardness along the axis of the target are well within the acceptable tolerance range for nonsemiconductor applications of ion implantation when the ratio of length to diameter does not exceed 10.