Abstract: About oneyear characteristics of the mixed layer was observed by an eyesafe, compact Micro Pulse Lidar (MPL) at Peking University. A retrieval method of the Mixed Layer Height (MLH) which reduces error of instrument correction is presented. Three case studies were carried out using the data observed on 24 Mar, 4 Apr and 18 Apr 2002. Comparisons were made between the new method calculations, the results of the retrieval method developed by Hoff et al. (1996) and the raw insonde data. The results showed that MLH derived from lidar data using the norm alized first derivative method agrees well with that from rawinsonde data. The c orrelation coefficient and rms errors that characterize the MLH retrievals with the new method and the rawinsonde data in Beijing were 0.94 and 99.4 m; with the first derivative method and the rawinsonde data, they were 0.47 and 310.4 m, re spectively. MLH and its growth rate under clearness and cloudless condition were estimated from the MPL measurement. The characteristics of diurnal and seasonal variation of MLH were derived with the data set，including the relation between the aerosol mass concentration and MLH, the diurnal variation of ventilation co efficient. The results were as follows: Annual variation of monthly mean maximum mixing depth (MMD) and the growth rate accorded with a double cycle pattern, th at was, the minimum value of monthly mean MMD and growth rate occurred in October, but the maximum value of them occurred in May and September. The phase of annual variation of monthly mean growth rate was almost consistent with that of annu al variation curve of monthly mean MMD. This showed that their positive correlation was remarkable. However, the annual variable tendency of the time mixing depth reached maximum was different from that of monthly mean MMD. In other words, the surface heat flux was a primary factor affecting MMD. The development of the mixed layer exhibited temporal behavior similar to that of the aerosol optical thickness calculated from the lidar measurement using the Klett method. It can be concluded from the diurnal variation of ventilation coefficient that the varia tions in ventilation coefficient followed more closely with the variations in av erage wind than those in mixing depth. Some relations between several parameters about mixed layer were obtained from backscatter of lidar. This work also demon strated that a MPL is quite useful and predominant for long-term monitoring of the urban mixed layer.