In high-power microwave diode design, the space-charge-limited current is important because of its relation to the diode property and the formation of the virtual cathode. Although the Langmuir–Blodgett law, as a numerical solution, is helpful, a simple functional expression is more convenient for practical research and can avoide the divergence of the series for large R\-c/R\-a. First principle is used to generalize two-dimensional space-charge-limited current for planar plane capacitor and coaxial cylinders, and its reliability has been examined adequately. In this paper we obtain the functional expression of space-charge-limited current for concentric sphere, and the space-charge field factor in this expression by comparing the classical Langmuir current with the current from concentric sphere.