Random modulation refers to the changing of the MOSFET threshold voltage cell by cell. This paper claims it is essential in sub-2-V CMOS design because it reduces the sub-threshold leakage current even in the active and sleep modes as well as in the stand-by mode. We found that a gradated modulation scheme, which gradually changes the ratio of low- Vth cells according to the path-delay, is the best approach. To achieve the minimal leakage current, the way of determining the optimum pair of threshold voltages is also described. Experimental results for microprocessor show that gradated modulation reduces sub-threshold leakage current by 75% to 90% compared to conventional single-low-threshold voltage design without degrading the performance of the circuits.