The observation of particle number concentration and size distribution has been conducted for two weeks in May of 2006 in the downtown Ji’nan. It has been found that the number concentration of particles smaller than 200nm got the half hour average concentration of 10500cm-3. The percentage of the number concentration of ultrafine particles (Dp<100nm) was found to be 95%, which is much higher than what have been observed in other continental sites. The relative clean air condition with high temperature and low relative humidity provided the favorable condition for the formation and growth of ultrafine particles. And the nucleation processes were thought to be promoted mainly by homogeneous nucleation and heterogeneous condensation of precursor gas or vapors on pre-existing particles. The diurnal variations of particle number concentration and relative pollutants were also studied. High consistency of profile of SO2 and particles smaller than 200nm proved that SO2 played a key role in new particle formation. The particles whose diameter is larger than 200nm were thought to be caused by road dust.