The smectite-to-illite transformation has close relation with sediment and thermal history of basins. So illite can indicate period of oil formation. Dating of authegenic illite is an important task. K/Ar  method and  ⁴⁰Ar/³⁹Ar method are often applied to illite dating. The contamination of detrital illite hinders the direct application of isotope ages. In order to separate authegenic illite from detrital, IAA and alkylammonium methods are performed in illite separating. Before using IAA method, the samples must be separated by size. XRD spectrum shows that the content of authegenic illite is high in fine sample and the detrital illite is the major component in coarse sample. Each ages can be obtained respectively by K/Ar method. The above ages and the percentage of detrital illite have a linear relation. Authegenic and detrital ages can be calculated by extrapolation. Alkylammonium cations method is a new way to research the diagenetic age of sedimentary rocks. Alkylammonium cations can replace K cations in mica and illite stoichiometrically. Longer chain alkylammonium and more time replace more K cations than shorter chain and less time. Moreover, HRTEM observations indicate that the process is very selective: even in apparently homogenous illite or I/S, some crystals undergo opening and some remain unchanged. And the replacement of K cations by Alkylammonium cations is not restricted to crystal edges, but extends to entire interlayer. The above are the basic of alkylammonium cations method applied to the diagenetic age of sedimentary rocks. It is generally assumed that there are no preferential removal of K and radiogenetic Ar. A common opinion is that after alkylammonium treatment, it can be inferred that detrital component are opened preferentially if K-Ar age decreases. Between diagenetic and detrital illites, alkylammonium reacts with detrital illite first. Some other approaches such as vacuum encapsulation are applied, which improve the effect of dating. Because the Argon's escape from authegenic and detrital illites need different temperatures, step heating method can separate two kinds of illites. This article briefs the present progresses of illite dating among the world.