In forest inventory, traditional inventory methods that have been applied to study on rare and/or valuable species are confronted by a number of real issues such as no or limited number of target species in a sample plot, as a result, analyzed results do not reflect the true image of theserare species in a particular stand. In dealing with these issues, an inventory method has been recommended by Thompson in 1990 (so-called Adaptive Cluster Sampling) by creating sample plots in cluster form that can be adapted with this distribution character of species in the stand. According to the method, if an initial plot that satisfies with a pre-identified C-condition, an expansion of inventory procedure following 4 directions will be applied. This procedure will only be ended when C-condition is not met In order to help readers to follow and apply the method in their studies, this paper is going to introduce about ACS method in which it will emphasis on applied objects, sampling methods, sample plot arrangement, equations and particular samples.