Abstract: Abstract Ionospheric oblique sounding is a powerful tool for acquiring the status of the ionosphere. An oblique ionogram is obtained by using oblique sounding, which contains useful information about the ionosphere and can be used to achieve the ionospheric variation characteristic of given oblique link. Take Suzhou-Qingdao and Lanzhou-Qingdao links for example, the dinural variations of the Maximum Observed Frequency of the ionospheric F layer (MOFF) is analyzed; Observational data of the MOFF obtained by 12 ionospheric oblique links in China during 2013 were used to study the variation characteristics of their monthly medians in winter, summer and equino. It shows that the MOFF of each link present obvious diurnal and seasonal variations as the ionospheric electron concentration. The MOFF data obtained in 2013 by Qingdao-Xi'an and Guangzhou-Haikou links were chosen for further study by comparing with the equivalent MOFF data calculated by the vertical sounding data near the mid-point of the chosen links. The preliminary results indicate that MOFF has good agreement with the calculated equivalent one, which indicates its value for short-wave frequency selection or ion-ospheric environment studies.