Monocular residues in tobacco leaves were analyzed by high performance gas chromatography. In this method residues were extracted with HCL and transformed into Nereistoxin in alkali liquid, then calculated the content of Monocular in tobacco. This study confirmed the optimum condition of transform efficiency of Nereistoxin through changing the pH, temperature and time. It demonstrated that the optimum condition was pH 9.0-9.5 and water bath temperature 70 ℃ for 2 hours. In this condition, recoveries in tobacco leaves was 87.74%-91.44%, and relative standard deviations was 3.69%-5.73%. This method meets the requirement in term of sensitivity and recovery.