- This research project studied potential future changes of near-surface air temperature (shortly, temperature) and precipitation over Thailand due to global climate change, using regional climate model RegCM3. The model has long been developed and continuously updated, and it has been widely used by scientists worldwide. In this study, regional climatic conditions were simulated to a grid horizontal resolution of 20 km, spanning two 40-year periods: past-year period (1961-2000) and future-year period (2031-2070) under projected future emission scenario A1B of the Intergovernmental Panel on Climate Change (IPCC). To evaluate the overall performance of RegCM3 configured and applied here, simulated results were compared against observation data from a number of surface monitoring stations of the Thai Meterological Department. It was found that the model suffers from substantial underestimation on (40- year average) temperature for most of the country (except for Southern region with a lesser degree), with mean bias over several areas being as large as -4°C. However, the model was satisfactorily capable of capturing the annual seasonality and geographical variations of those two variables. For (40- year average) precipitation, the model tends to give overestimates in Southern and Central-Eastern regions and underestimates in Northern and Northeastern regions. The spatial distributions of (40-year average) temperature simulated for the future years are similar to those simulated for the past years. In the future, temperature is expected to increase over the entire Thailand by <1-3°C, and its trend is positive (i.e., increasing) in every region along the future decades in question. For (40-year average) precipitation simulated for the future years, most areas would not have substantial change in terms of precipitation amount when compared to the past years, N except in Southern region where a decrease of 3-5 mm/day is predicted. For selected eight climate extreme indices, it was found for the future years as follows: For Hot Days Index (TX90p), it tends to incease along the future decades over the country. For Cool Days Index (TX10p), the direction would be opposite of that of TX90p. For Warm Nights Index (TN90p), the direction is the same as TX90p. For Cold Nights Index (TN10p), the direction is similar to that of TX10p in that the index is predicted to decline with the decades in all regions. For Significant-Precipitation Days Index (R10mm), no substantial differences (between the past years and the future years) were found in every region, except in Southern and Eastern regions. For Consecutive Dry Days Index (CDD), many areas in Northeastern region (particulary in its central part) would see an increase in this index while the wertern part of Northern region would see a slight decrease. For Consecutive Wet Days Index (CWD), no substaintial differences were found across Thailand, except in Southern region where the index declines. For Precipitation Due to Very Wet Days Index (R95p), no substantial differences were also found across Thailand, except in Southern region and also Eastern region where the index declines.