To evaluate the acute effects of fine particles on pulmonary function, a longitudinal study was conducted. This study was carried out for the schoolchildren (3rd and 6th grades) living in Beijing, China. Each child was provided with a mini-Wright peak flow meter and a preformatted health symptom diary for 40 days, and was trained on their proper use. Participants were instructed to perform the peak flow test three times in standing position, three times a day (9 am, 12 pm, and 8 pm), and to record all the readings along with the symptoms (cold, cough, and asthmatic symptoms) experienced on that day. Daily measurement of fine particles (PM$_{10}$ and PM$_{2.5}$) was obtained in the comer of the playground of the participating elementary school for the same period of this longitudinal study. The relationship between daily peak expiratory flow rate (PEFR) and fine particle levels was analyzed using a mixed linear regression models including gender, height, the presence of respiratory symptoms, and daily average temperature and relative humidity as extraneous variables. The total number of students participating in this longitudinal study was 87. The range of daily measured PEFR was 253-501$ell$/min. In general, the PEFR measured in the morning was lower than the PEFR measured in the evening (or afternoon) on the same day. The daily mean concentrations of PM$_{10}$ and PM$_{2.5}$ over the study period were 180.2$mu extrm{g}$/㎥ and 103.2$mu extrm{g}$/㎥, respectively. The IQR (inter-quartile range) of PM$_{10}$ and PM$_{2.5}$ were 91.8$mu extrm{g}$/㎥ and 58.0$mu extrm{g}$/㎥. During the study period, the national ambient air quality standard of 150$mu extrm{g}$/㎥ (for PM$_{10}$) was exceeded in 23 days (57.5%). The analysis showed that an increase of 1$mu extrm{g}$/㎥ of PM$_{10}$ corresponded to 0.59$mu extrm{g}$/㎥ increment of PM$_{2.5}$. Daily mean PEFR was regressed with the 24-hour average PM$_{10}$ (or PM$_{2.5}$) levels, weather information such as air temperature and relative humidity, and individual characteristics including gender, height, and respiratory symptoms. The analysis showed that the increase of fine particle concentrations was negatively associated with the variability in PEFR. The IQR increments of PM$_{10}$ or PM$_{2.5}$ (at 1-day time lag) were also shown to be related with 1.54 $ell$/min (95% Confidence intervals: 0.94-2.14) and 1.56$ell$/min (95% CI: 0.95-2.16) decline in PEFR.R.ine in PEFR.ine in PEFR.