This study revealed great potential and shortcoming of offshore wind energy in Vietnam by numerical simulations with Weather Research and Forecasting (WRF) model at 10-km resolution for 10 years (2006-2015). The greatest energy potential was found in the offshore area of Phu Quy island (Binh Thuan province). The area, alone, can provide the 38.2 GW power generation capacity corresponding to the increasing renewable-energy demand by 2030 planned by the country. There is also a drawback of the wind resource, which is associated with strong multiple-scale temporal variabilities. The seasonal variability associated with monsoon onsets and daily variability associated with the wind diurnal cycles were found ranging 30-50%. Meanwhile, the inter-annual variability could reach up to 10%. These variabilities must be considered when designing wind farms and grids over the region. Additionally, due to the fact that the WRF model performed climatological features of the winds well against the observations, this results indicate that it can be useful tools for wind-power assessment as compared to other reanalysis or QuikSCAT data with coarser spatio-temporal resolutions.