Ring‐opening isomerization from ring‐shaped isomers to chain‐shaped isomers of N8H8 has been studied by a density function B3LYP method at 6‐311++G** level. 20 ring‐shaped isomers have been found to be able to transform into chain‐shaped isomers, with 20 possible transition states got by ring‐opening structure optimization. Furthermore, the ring‐openings have been found in the longer NN single bond by analyzing the length change of NN bond of ring‐shaped isomers in ring‐opening processes. In addition, with the activation energies in ring‐opening processes, the differences of the activation energies in isomerization between the isomers have been found according to the classification of rings. The activation energies in ring‐opening isomerization of six‐membered ring‐shaped isomers are higher than that of the four‐membered ring‐shaped isomers. It indicates that six‐membered ring‐shaped isomers difficult in ring‐opening in the isomerization are the steadiest ring‐shaped isomers of N8H8 while four‐membered ring‐shaped isomers easy in ring‐opening are the most unstable. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011