The planetary boundary layer (PBL) structures on four light wind days at Suzhou city, China were analyzed with both micropulse lidar (MPL) and passive microwave radiometer (MWR). The planetary boundary layer height (PBLH) was estimated with the first-derivation method from the lidar observations. The lidar-observed PBLH and microwave radiometer observations were used to estimate the lapse rate in a simple encroachment model. Under light wind conditions in this area, the nocturnal PBLH is about 300 to 400 m. The maximum of PBLH was about 800 to 1000 m. The daytime PBL development could be divided into three periods: growing, maintenance, and decay. The PBLH is nearly linear in the growing period with a growth ratio of about 110 to 120 m/h. The encroachment model lapse rate was calculated from the MPL and MWR observations. The lapse rate γ is estimated in the four days and the average is about 0.0010 ?startKend?/m. The vertical structure of air temperature and humidity were also analyzed with the microwave radiometer observations. Diurnal variations of relative humidity in the boundary layer partly explain the light fog appearing at night during this period.