This study aimed to determine suitable C/N ratios for larval rearing of giant freshwater prawns in a biofloc system. There were 5 treatments (1) C/N=10, (2) C/N=20, (3) C/N=30, (4) modified-static green water system (control), (5) open-clear water system (control). At the beginning, biofloc rearing tanks were inoculated by biofloc water from tilapia culture. Stocking density was 90 larvae/liter, and rearing tank volume was 50L/tank. During rearing period, in biofloc treatments, rice flours and soybean flours were added with suitable ratios to reach C/N ratios are 10, 20 and 30, respectively, in which total concentration was 20ppm/3days and water was not exchanged. Water quality parameters (temperature, pH, DO, TAN, nitrite, alkalinity), floc parameters (floc volume index FVI and floc composition), microbiological analysis, growth in length, larval stage index, survival rate and productivity were monitored. The results showed that FVI ranged from 1.8±0.7 to 5.4±2.3 ml/L and was largest in treatment (3), significantly different compared to the other treatments (p