Abstract: The research is to determine diffusion coefficients of))~237)Np in three kinds of bentonite under atmosphere and anoxic atmosphere by back-to-back method. Da values determined by back-to-back way as follows:the mixed bentonite is 3.9×10~-13) m~2/s, Mg-bentonite 8.4×10~-13) m~2/s,Ca-bentonite 5.6×10~-13)m~2/s,respectively;and corresponding Da values under anoxic atmosphere:3.4×10~-13) m~2/s,3.1×10~-13) m~2/s and 4.7×10~-13) m~2/s. On the other hand,the Da value of))~237)Np on mixed bentonite is determined by flow experiment,and the result is Da=8.2×10~-12) m~2/s.