Designing MOS decoupling capacitors (DECAPs) in nanotechnologies provides many challenges due to the existing trade-offs among transient time response behavior, area, and gate leakage current. In this paper first it is shown that all of these challenges are functions of the MOS DECAP channel length. Then, we propose a method for optimizing the channel length of MOS DECAPs. The technique is applied to 45nm and 32nm technology nodes and the results are extracted using HSPICE simulations. Also the accuracy of the proposed technique is verified. Finally, based on the results, two optimum DECAP configurations which provide trades off among area and gate leakage for different applications in nanotechnologies are proposed.