Using etch-figure technique, we have directly observed dislocation structure of sub-boundaries in electron-beam zone-melted molybdenum single crystals. A thorough analysis of experimental results are given, these are further compared with the deductions derived from the Frank's formula for subboundaries, and the theoretical predictions are fully verified.The directions of parallel etch-lines observed on (111) plane are along the intersection lines of glide planes and climb plane, it proves that these are symmetrical tilt subboundaries composed of edge dislocations with Burgers vector 1/2. The verification of the generalized Read-Shockley formula for intersecting subboundaries revealed by etch-pitting (110) surface proves these to be asymmetrical tilt subboundaries composed of two sets of dislocations.15 sets of networks of etch-lines have been analyzed, in which 5 sets arc proved to be 1/2 / networks, 9 sets to be / networks. In the network analysis, in addition to the stereographic method used by Carrington ct al., we have developed a new method by verifying Frank's formula quantitatively and it proved to be effective in determining Burgers vectors of dislocation networks when the stereographic analysis failed to yield unique result.Besides, we have observed various patterns indicating interactions of singular dislocations with subboundaries, in particular, "steps" on the subboundaries induced by singular dislocations and also interactions of inclusions with subboundaries. Various types of subboundary junctions and non-equilibrium subboundaries have been observed and discussed.All these results prove that the etch-figure method of observing dislocations is an effective means for the quantitative study of dislocation substructure in crystals, comparable in power with that of transmission electron microscopy.