The authors have studied the decomposition of monazite concentrate by alkali. They investigated the necessary decomposition conditions such as temperature, time, particle size and the ratio between alkali and monazite concentrate by mass (wt/wt). The decomposition is best performed at >140°C, within 8 hours, the required alkali and monazite concentrate ratio was 1,4/1, the ore particle size needs to be smaller than 48 mm. The decomposition efficiency was also only about 70%. If the desired recovery efficiency was higher than 90%, the alkali/concentrate ratio by mass needed to be at least 4/1, resulting in a large amount of residual alkali. To improve this process, the authors studied the decomposition of monazite concentrate by alkali under pressure. The monazite was taken from Ham Tan deposit. The effecting parameters such as temperature, time, ratio between NaOH/concentrate by mass and particle size of the concentrate were investigated. The results showed that, under the effect of pressure, decomposition occurs faster, more thoroughly and decomposition efficiency increases. The suitable temperature for concentrate decomposition was from 180 to 210°C, corresponding to a pressure of 4 to 7 at. The decomposition time was also reduced to 2 hours and the particle size of the concentrate was also larger to 55 mm with 70% alkali and ratio of alkali/concentrate was 1/1. The decomposition efficiency of rare earth elements reached 95%, while under the same conditions, U was 50% and Th was 77%. When the reaction time increased, the efficiency of rare earth elements decomposition did not change much, but the decomposition efficiency of radioactive elements tended to increase. For particle sizes >55 mm, the process efficiency decreased sharply, and changes in reaction conditions were needed to increase the decomposition efficiency.