Abstract: In this paper, Es occurrence and f₀E over Chongqing (29.5°N, 106.6°E) and Lanzhou (36°N, 103.9°E) are compared and studied. The results show that: (1) Change pattern of Es occurrence over these two stations has certain rules. During the daytime, Es occurrence over Chongqing is higher than that over Lanzhou, but at night the result is just the opposite. (2) Change of Es occurrence is consistent to that of the ambient ionization density. (3) Besides the difference of latitude of the two stations, it seems that the mechanism of the above characteristic is closely related to the S_q current systems.