An analytical study is presented for the Coulomb staircase in the I-V curve of an ultrasmall double-barrier resonant-tunneling system. Below a certain threshold voltage V 1, the charging effect of a single electron in the ultrasmall confinement makes the tunneling current I 0= 0 at very low temperature. As the voltage V rises above V 1 but below V 2 at which the charging energy is overcome, the first step of current I 1 shows up. When V becomes even greater than V 2, both the charging energy and the Coulomb repulsion between two electrons of opposite spin are overcome and the current jumps onto the second step I 2. The ratio between the two current steps (I 2-I 1)/(I 1-I 0) depends upon the ratio of the tunneling rates of the two barriers. Our results agree well with a recent experiment by Su, Goldman, and Cunningham.