The study on treatment capacity ofwater lettucefor wastewater from Thai frog farm was arranged in a completely randomized designwith 4 coverages: 0% (control treatment),50%, 75%, and 100% of plants, 3 replications for each treatmentin 8 weeks. The water samples were collected at 24, 48, 72 hours interval to determine the retention time, and at 2, 4, 6, and 8 weeks to evaluate the wastewater treatment capability of water lettuce by phase growth. The results showed that the quality of outlet water in plantedtreatments met the A column of QCVN 40/2011/BTNMT after 72 hours. The concentrations of N-NH4+, P-PO43-and COD met A column of QCVN 40/2011/BTNMT and concentrations of N-NH4+, N-NO2-and N-NO3-met column A1, P-PO43-and COD met B2 column of QCVN 08/2015/BTNMT after 4 weeks. The treatment efficiency of N-NH4+, N-NO2-, N-NO3-,TN, P-PO43-, TP and COD was 28.5-58.6; 39.4-59.8; 56.9-71.6; 32,1-58.7; 64.9-80.1; 43.7-71.8 and 57.4-79.8%, respectively, which increased and had positive correlation with the density of plants. Water lettuce helped to increase the treatment efficiency of frog farming wastewater and shoot and biomass grew well, however, plants were dead in 75% and 100% treatments from 6 weeks due to high density, so it was advisable to harvest a part of biomass to avoid re-release of nutrients from decomposing plants.