Abstract: This paper presents the results of HF channel time delay measurements on 580 km, 940 km and 1 380 km paths using oblique ionospheric sounder equipment, and gives a curve fitting data processing methods to deal with measurements data of single mode. The variety characteristics of single mode FRTD is discussed on the basis of analysis of calculating results, it indicates that the FRTD values decrease with frequency interval getting small and paths distance increasing, the maximum FRTD value in a day appears at noon.