Abstract: The 11-years'LF phase data at noontime at Wuchang are used to analyse seasonal variations of the D region. The results show that the reflection height of LF in mid-and lower latitudes has a dominant annual variation with the minimum in summer and the maximum in winter, i.e. the electron density of D region is obviously higher in summer than that in winter, which is a little different from that in the mid-and high latitude ionosphere. The amplitudes and phases of this annual variation for each year are calculated. The day with zero degree phase is close to the winter solstice, which suggests that the main reason of the annual variation is the seasonal variation of the zenith angle. And the amplitude due to zenith angle variation is estimated in this paper.The fluctuations of amplitudes of annual oscillation show an interannual variability with little relation to the solar 11-year cycle, but seems to be modulated by a quasi-two year oscillation.