New findings of ammonites in Oxfordian on the Chernokhrebetnaya river (East Taimyr) allowed elucidation of the sequence of species of Cardioceras and Amoeboceras, which is very similar to that established in the middle and upper substages of Oxfordian in Scotland and East Greenland. These sections are recognized as stratotypical for the boreal Middle and Upper Oxfordian. In the Middle Oxfordian of the east Taimyr there are zones of densiplicatum and tenuiserratum, in the Upper Oxfordian - Amoeboceras glosense zone with subzones of ilovaiskii and glosense; zones of serratum, regulare and layers with Amoeboceras ex gr. rosenkzanti. The Siberian zonal scheme correlates well with the zonal boreal standard. The zonal scheme for the Middle and Upper Oxfordian of the east Taimyr is determined as type for the whole East Siberia. A bed-by-bed description of Oxfordian on the Chernokhrebetnaya river is reported. A plate representing the most important species of the Middle and Upper Oxfordian Cardioceras and Amoeboceras is given.