In order to understand the seasonal variation of aerosol optical properties in the Yangtze River Delta,5 years of measurements were conducted during September 2005 to December 2009 at Taihu,China.The monthly averages of aerosol optical depth were commonly ＞0.6; the maximum seasonal average (0.93) occurred in summer.The magnitude of the Angstr(o)m exponent was found to be high throughout the year;the highest values occurred in autumn (1.33) and were the lowest in spring (1.08).The fine modes of volumesize distribution showed the maxima (peaks) at a radius of ～0.15 μm in spring,autumn,and winter; at a radius of ～0.22 μm in summer.The coarse modes showed the maxima (peaks) at a radius of 2.9 μm in spring,summer,and autumn and at a radius of 3.8 μm in winter.The averages of single-scattering albedo were 0.92 (spring),0.92 (summer),0.91 (autumn),and 0.88 (winter).The averages of asymmetry factor were found to be larger in summer than during other seasons; they were taken as 0.66 at 440 1020 nm over Taihu.The real part of the refractive index showed a weak seasonal variation,with averages of 1.48 (spring),1.43 (summer),1.45 (autumn),and 1.48 (winter).The imaginary parts of the refractive index were higher in winter (0.013) than in spring (0.0076),summer (0.0092),and autumn (0.0091),indicating that the atnosphere in the winter had higher absorbtivity.