We have made direct observations of dislocation substructure in LiNbO3 and LiTaO3 single crystals decorated by gold particles by mean of transmission optical microscopy and ultra-microscopy. The three dimentional features of the subboundaries have been disclosed and the nature of subboundaries are determined with stereographic analysis, and quantitatively verified by Frank's formula.The analysis of dislocation networks and junctions of subboundaries shows that the Burgers vectors of dislocations in these crystals are , the smallest lattice vector;1/3, the next smallest lattice vector, and 1/3 , the short diagonal of therhombohedral cell.