Level shifters form an indispensable part of modern system on chips (SoCs). Traditional figure-of-merit(FOM) of energy-delay product is not appropriate to characterize level shifters used in certain charge constrained applications. This paper proposes a new FOM of charge-delay product to characterize the level shifters. Some existing architectures have been discussed on the basis of this new FOM. A new architecture to improve the charge-delay product is also proposed. The new circuit shows that the FOM improves from 807fC/MHz to 155fC/MHz, an improvement of about 80%.