In this paper, we propose an application-driven ALU design methodology to achieve high level of power efficien-cy for modern microprocessors. We introduce a PN selection algorithm (PNSA) which enables designers to select power efficient dynamic modules for different applications, based on the detailed analysis of dynamic circuits. Experimental results on ISCAS85 and 74X-Series benchmark circuits show that the power consumption of 8-bit ALU based on this approach can be reduced by 54%-60% for different frequency levels as compared to the conventional dynamic ALU design, demonstrating the effectiveness of the pro-posed method on application-driven custom ALU design.