Models of trapezoidal barrier in two extreme cases are used in this paper to study barrier parameters of A1-A12O3-M tunnel junctions, the top electrodes of which are Au, Ag and Cu, respectively. From the fitting calculation of V-I characteristics of the above junctions at 77 K, we find that both models can explain the experimental results very well, and the barrier parameters correlate with the atom radius of top electrode metal, the work function of top electrode metal material and the electron affinity to isolarer medium A12O3, respectively.