Based on the sequence stratigraphic analysis of the transitional deposits of Cambrian-Ordovi-cian boundary in North China Plate, are recognized three depositional sequences:the Fengshan depositional sequence (DS F) ,the Lianghekou-Honghuayuan depositional sequence (DS LH) and the Dawan depositional sequence (DS D). The boundary between DS F and DS LH is considered as the I -type sequence boundary (SB2) ,and the boundary between DS LH and DS D the I -type sequence boundary (SBl). The Cambrian-Ordovician boundary should be placed at the bottom of the TST of the DS LH, i. e. between the micrite mound of Jiuyuan Fm. and Yeli Fm. in Western Hills, Beijing and between the dolostone of Sanshanzi Fm. and Hanjia Fm. in Xuzhou area.