The solar ultraviolet (UV) index from Ozone Monitoring Instrument (OMI) onboard NASA EOS/Aura satellite is compared with that derived from ground-based multi-channel filter radiometers at four sites in the tropical environment of Thailand. The sites include Chiang Mai (18.78°N, 98.98°E), Ubon Ratchathani (15.25°N, 104.87°E), Nakhon Pathom (13.82°N, 100.04°E) and Songkhla (7.2°N, 100.6°E). At these sites, aerosol optical depth is monitored using sunphotometers and cloud images are taken by sky cameras. The comparison results clearly show the overestimation of OMI UV index because of influences by aerosols and clouds. The differences between the two datasets lie between 38.1 and 60.7% for all sky conditions, and reduce to 22.5–50.3% for clear sky conditions.