The aerosol number concentration and size distribution were measured with the newly developed Wide-range Particle Spectrometer in summer and winter of 2006 at the urban site of Ji’nan city. Here reported the characteristics of fine particles of the different observation seasons. Relative high number concentrations for the particles in the diameter range of 10-500 nm were observed in both seasons. It was found that the dominate number distributed in particle diameter smaller than 100 nm and the percentage over the number concentration of all air particles is much higher than what has been measured in other urban sites over the world. The number mean diameter in summer was much smaller than in winter, strongly suggesting the different origin of ultrafine particles in different seasons. That is, particles in ultrafine mode mainly came from nucleation and new particle formation in summer while from traffic emission in winter. The dial variation also supported this point. Number concentration in the diameter range of 10-200 nm got their peak values at noontime, well correlated with the mixing ratio of SO2 and the intensity of solar radiation in summer. While in winter, those in the same diameter range showed the main peaks during the traffic hours happened in the ming and evening.