The formation and structural stability of defect-free non-orthogonal X-junctions, composed of ultrathin SWCNTs (UTCNTs) with varying angles, through heat treatment is investigated by molecular dynamics simulation. It is found that non-orthogonal X-junctions of UTCNTs are prone to transform into parallel junctions (P-junctions), and further coalesce into a SWCNT, when heated at a high temperature. Variation of the internal torque of the UTCNTs during this transformation is analyzed, and the formation energy for the final coalesced structure is also discussed. It shows that the transformation is a result of the torque originating from the heating of the non-orthogonal X-junction. (C) 2012 Elsevier B.V. All rights reserved.