The decommissioning cost of a nuclear power plant (NPP) is largely composed of activitydependent costs, period-dependent costs, and collateral costs. And activity-dependent costs for each decommissioning activity are composed of five cost elements: Removal, decontamination, packaging, shipping and disposal. Among these, the removal cost elements are calculated by multiplying the appropriate inventory data element by the corresponding unit factors (UF), which are developed in terms of labor hours to perform an activity on a per unit basis. The labor hours included in UF is calculated under theoretical working conditions, which, after being multiplied by labor rates, composes unit cost factor (UCF) along with material cost. In the actual working conditions, there are number of factors that increase the time needed for performing a task. The effects of these factors are taken into consideration by means of work difficulty factor (WDF), expressed as a percentage of increase of the working time, comparing to an unimpeded working situation. WDF, by increasing the labor hours and consequential labor cost in UCF, makes it possible to calculate the actual removal cost. There are about five types of adjustment factors commonly used as WDF: Height, Respiratory protection, Radiation, Protective clothing, Work break. Considering the different working conditions, all of the five factors’ combination could be used theoretically, which results in the huge increase of the number of WDFs. For practical purpose, two representative WDF application methods has been used in the dismantling decommissioning cost evaluation program: A separate development of the UCFs, WDFs applied to the decommissioning area. In the first method, all of the UCFs, having different working environment, should be developed separately by the cost estimator. In the second method, UCFs are to be allocated to the relevant decommissioning areas where WDF sets are predefined by the cost estimator. In this study, the components of the decommissioning cost, the relation between UCF and WDF, and WDF application methods were reviewed. The result of review implies that WDF has a great influence on decommissioning cost. Additionally, since WDF application methods have somewhat limitations and complexity, their characteristics should be sufficiently examined by the user before being used.