Aerosol optical thickness (AOT) retrieved by the Multi-angle Imaging SpectroRadiometer (MISR) from 2002 to 2004 were compared with AOT measurements from an Aerosol Robotic Network (AERONET) site located in Beijing urban area. MISR and AERONET AOTs were highly correlated, with an overall linear correlation coefficient of 0.93 at 558nm wavelength. On average, MISR AOT at 558 nm was 30% lower than the AERONET AOT at 558 nm interpolated from 440 nm and 675 nm. A linear regression analysis using AERONET AOT as the response yielded a slope of 0.58 and an intercept of 0.07 in the green band with similar results in the other three bands, indicating that MISR substantially underestimates AERONET AOT. After applying a narrower averaging time window to control for temporal variability, the agreement between MISR and AERONET AOTs were significantly improved with the correlation coefficient of 0.97 and a slope of 0.71 in an ordinary linear least squares fit. A weighted linear least squares, which reduces the impact of spatial averaging, yielded a better result with the slope going up to 0.73. The best agreement was achieved with the slope of 0.91 when only the central points are Abstract included in the regression analysis. By investigating PM10 spatial distribution of Beijing, we found substantial spatial variations of aerosol loading, which can introduce uncertainty when validating MISR AOT. Our findings also suggest that MISR aerosol retrieval algorithm might need to be adjusted for the extremely high aerosol loadings and substantial spatial variations that it will probably encounter in heavily polluted metropolitan areas.