This study aims to evaluate the effects of climate change and El Ni?o–Southern Oscillation (ENSO) on reference evapotranspiration (ETo) in the climate sub-regions of South Vietnam (SVN) and the role of the related meteorologi-cal variables. The trend analysis shows that ETo has been increasing quite clearly, especially during the rainy season. By examining the contribution of meteorological variables to the increase of ETo, it is indicated that temperature is the main contributor. Among the meteorological variables that are related to ETo, temperature is also the factor that has gained the most significant increase. Analysis of variance reveals that there is no difference in the increase of ETo among the climate sub-regions of SVN. The effect of ENSO on ETo is assessed based on the difference in the mean value of ETo between the El Ni?o and La Ni?a phases. The results show that this difference is most obvious from Oc-tober to May, and the main factor contributing to the increase is not temperature but sunshine hours. The difference in sunshine hours between the warm and cold phases in these months is around one hour per day, contributing about 58％–86％ to the ETo difference. Further analysis of variance shows that ENSO has different levels of influences on ETo in the climate sub-regions. Compared to the increase in ETo due to climate change over the past 40 years, the ETo difference between El Ni?o and La Ni?a phases is many times higher. In addition, since the effect of ENSO on ETo is most obvious in the study area during the dry season, it is much stronger. In order to mitigate the effect of EN-SO on drought in this area, monitoring and forecasting meteorological variables that have the main contribution to the variation of ETo, including the number of sunshine hours, should be promoted.