The use of urease enzyme inhibitor for reducing the loss of NH3 in rice fields is an important process. In this study, two urease inhibitors which are phenyl phosphorodimidate (PPD) and N-(n-butyl) thiophosphoric-triamide, (NBPT) at different regimes 0.5 percent
1 percent and 2 percent were used on Than Nong Tet rice cultivar. After the treatments with inhibitor supplements, basically, the urea remaining in fields was higher than the control fields (without PPD and NBPT). The supplementing use of PPD and NBPT helps save 20 percent of the nitrogen fertiliser in the field without changing the harvest. The optimum percentage of PPD and NBPT is 0.5 percent.