Quantum-dot Cellular Automata (QCA) is potentially a very attractive alternative to CMOS for future digital designs. Circuit designs in QCA have been extensively studied. QCA technology has been extensively investigated in recent years. However, only limited attention has been paid to QCA decimal arithmetic. In this paper, an efficient full adder is presented. The layout of the proposed circuit uses 30 QCA cells and a 25% improvement over the best previous design. It has significant improvements in comparison to the previous designs in terms of the number of cells, area and cost and has a similar delay to the fastest previous design. In Ripple Carry Adder (RCA) design, they also maintain high performance. And all of the multi-bit RCA also have the lowest overall cost with a reduction of over 50% when compared with the previous RCA design.