By using the eight-times-daily sampling data from an intensive radiosonde observation campaign at Yichang(111°18′E,30°42′N),China in August 2006 and January 2007,the diurnal variation of the planetary boundary layer height determined by using a bulk Richardson(Ri)number approach,was studied in this paper.It was found that the boundary layer heights in both summer and winter months showed diurnal changes and the daily cycle was deeper in summer,which agreed well with the previous studies;the monthly averaged height was 103–1112 m and 89–450 m in summer and winter,respectively;the morning rise began at 0700 LT/1000 LT in summer/winter and the evening transition occurred at 1900 LT in both seasons;the maximum height occurred in the afternoon for most cases,except some peaks found in the winter night;the surface temperature and relative humidity dominated the variations of summer height,while the diurnal variation shown in January 2007 might have some connections with the dynamical processes in the lower troposphere,besides the surface effects. By using the eight-times-daily sampling data from an intensive radiosonde observation campaign at Yichang (111 ° 18’E, 30 ° 42’N), China in August 2006 and January 2007, the diurnal variation of the planetary boundary level by using a bulk Richardson (Ri) number approach, was studied in this paper. It was found that the boundary layer heights in both summer and winter months showed diurnal changes and the daily cycle was deeper in summer, which agreed well with the previous studies the monthly averaged height was 103-1112 m and 89-450 m in summer and winter, respectively; the morning rise began at 0700 LT / 1000 LT in summer / winter and the evening transition occurred at 1900 LT in both seasons; the maximum the occurred temperature in the afternoon for most cases, except some peaks found in the winter night; the surface temperature and relative humidity dominated the variations of summer height, while the diurnal variation shown in January 2007 might have some connections with the dynamic al processes in the lower troposphere, besides the surface effects.