The chemical structure of carbon nitride thin films, prepared by rf-dc plasma enhanced chemical vapor deposition (PECVD) was studied by XPS. Analyzing the C(1s) and N(1s) core level lines indicated two types of chemical structure N-sp3C and N-sp2C binding states existed in the CN films. There is also little N-spC binding state in the CN film with a nitrogen content of 22%. The ratio of N/C in N-sp3C binding state is 1.28 which is near 4 : 3, thus demonstrated the existence of C3N4-like phase. High nitrogen content in the CN films is useful to increase the content of beta-C3N4 phase. Light changes of the chemical structure of the CN films can be observed under ion irradiation. With the increase of ion dose, the ratio of N-sp3C/N-sp2C increase, while the N/C ratio in the films decrease.