The potential energy surfaces involving 10 singlet minima and 28 transition states for the isomerization pathways of the HC3N isomers are explored by the scaled hypersphere search method at ab initio levels. In addition to seven known species, three new cyclic isomers, bicyclic-CCCN-H, C-cyclic-CCN-H, and cyclic-CCCN-H, are predicted at the CCSD(T)/cc-pVTZ//B3LYP/6-311++G(d,p)+ZPE level. They can interconvert to each other, isomerize to other low-lying isomers, or decompose into other fragments easily. All isomers can be considered as adducts via barrierless recombination channels between various fragments detected in interstellar space or favorable hydrogen-adding processes. (c) 2005 Elsevier B.V. All rights reserved.