Objective： To study NF-κB activation during the early phase of T cell reaction due to CD3／CD28 co-stimulation and the effect of arsenic trioxide on NF-κB. Methods： Hut-78 cells were stimulated with CD3／CD28 monoclonal antibodies（10 μg／ml and 5 μg／ml， respectively） in 30 min. TransAM was employed to investigate p65 expression in nuclear extracts and the effect of arsenic troxide. Results： The level of p65 in nuclear extracts of Hut-78 cells was much lower during the CD3／CD28 co-stimulation in 30 min， however arsenic trioxide seemed to inhibit the effect of CD3／CD28 co-stimulation. And the trend was nearly the same in whole cell lysates. Conclusion： NF-κB is suppressed during the early phase of CD3／CD28 co-stimulation in human T cells.