Abstract: The contents of free amino acids in brown planthopper (BPH) Nilaparvata lugens Stal were determined during its adaptation stage to resistant rice variety “IR26”. The results show that the contents of total free amino acids (TFAA) and essential amino acids (EAA) in leaf sheath and sap flow of “IR26” drop by 3.78% and 3.05% below those in the susceptible variety “TN1”. Brachypterous BPH female adults feeding on “IR26” obtains more TFAA and EAA, except phenylalanine and lysine in the second generation. However, TFAA and EAA contents decrease in the third generation. The contents of other free amino acids in female adult honeydew increase, except aspartic acid, glutamic acid and cystine, when BPH is transferred from “TN1” to “IR26”. When the BPH fed with “IR26” for three generations are transferred to “TN1”, the contents of threonine, valine, arginine, aspartic acid, serine, glutamic acid and glycine become lower than in those feeding exclusively on “IR26”. No obvious difference exists in TFAA contents between BPH feeding on “IR26” and “TN1”.