The Moderate Resolution Imaging Spectroradiometer (MODIS), an instrument aboard the Aqua platform, derives optical depth and other optical properties of aerosols. Aerosol optical properties retrieved at 32 Aerosol Robotic Network (AERONET) sites, the East Asian domain (90 E - 150 E, 20 N - 60 N) are used for validation of MODIS-derived aerosol optical thickness. For the comparison, MODIS-AERONET collocated optical thickness data were constructed, when two measurements are taken within 25 km distance and within a 30 minute time difference. All collocated data were averaged by applying distance-dependent weighing function. From the comparison of optical thickness between land and ocean, it was found that the correlation between MODIS and AERONET appears to be relatively high over the ocean, although there seem to be a bias over the ocean. In this paper, we discuss why the correlation is low over the land in comparison to the ocean.