Quantum-dot Cellular Automata (QCA) is a new and efficient technology to implement logic Gates and digital circuits at the nanoscale range. In comparison with the conventional CMOS technology, QCA has many attractive features such as: low-power, extremely dense and high speed structures. Adders are the most important part of an arithmetic logic unit (ALU). In this paper, four optimized designs of QCA serial adders are presented. One of the proposed designs is optimized in terms of the number of cells, area and delay without any wire crossing methods. Also, two new designs of QCA serial adders and a QCA layout equivalent to the internal circuit of TM4006 IC are presented. QCADesigner software is used to simulate the proposed designs. Finally, the proposed QCA designs are compared with the previous QCA, CNTFET-based and CMOS technologies.