This paper presents a new extraction method for effective channel length ( L _eff ) and source/drain series resistance ( R _SD ) in ultrashort-channel MOSFETs using an iterative process, which is a modified channel resistance method (CRM). Although conventional methods for extracting L _eff and R _SD , such as the channel resistance and shift-and-ratio methods, are considered to be the most consistent techniques, they are not valid for shorter channel transistors, such as the total resistance ( R _tot = V _DS / I _DS ) of MOSFETs, and do not scale proportionately with poly-gate length ( L _poly ). This error results from the fact that these methods assume the effective mobility f(mu _eff ) of long- and short-channel transistors to be the same. This assumption inevitably lowers the accuracy of the extracted channel length in ultrashort-channel MOSFETs. Therefore, an improved CRM using an iterative procedure has been proposed. This iteration method takes into account the fact that mobility is degraded in shorter channel devices. By compensating for the mobility in long-channel devices, more accurate approximations for L _eff and R _SD are extracted compared to conventional methods.