The radioassay of glutamic decarboxylase(GAD) was modified by taking NaOH as trapped agent instead of phenylethylamine. The results showed that the coefficient of variation (CV) within same sample was 9.6% and the radioactivity remains stable after 72 hours if use NaOH as trap agent. It is significantly stable than use phenylethylamine as trap agent, which the CV was 31.9% and the radioactivity decreased 47% within the first hour and decreased to background after 6 hours. The reabsorption experiment shows over 80% of ¹⁴CO₂ can be reabsorption by NaOH within 6 hours. It is suggested that NaOH is a much better trap agent than phenylethylamine and the sensitivity can increase 1.66 folds. Using this method the GAD activity in 0.39～17.8 mg of brain tissue can be measured and it is success in determine the GAD activity both in rat brain tissue and cultured neurons.