This paper presents the characteristics of the equatorial ionospheric F2 layer over Vietnam based on continuous monitoring data for 11 years from ionospheric station in Ho Chi Minh city (10.50'N, 106.33'E). These results determine the diurnal and seasonal variations with solar activity of the F2 layer. The ionospheric parameters were taken from ionograms are often used as the control elements in forecasting. The index representing levels of solar activity is the sunspot number R considered the basic indicators for modeling and forecasting the ionosphere. An important parameter is the maximum plasma frequency, because this is what determines which vertical transmissions will pass through and which will be reflected back. This maximum plasma frequency is called the critical frequency, fo. Because the plasma frequency varies for a given time and location, the critical frequency also varies. However the critical frequency is always within the HF range. This is what gives HF transmissions their unique characteristics. The highest plasma frequency is in the F layer (or more specifically the F2 layer). This critical frequency may be referred to using the symbol foF2.