Fusion reactions of size-selected linear polyyne molecules, C 10H2, in the ultrathin one-dimensional (1D) nanospace of double wall carbon nanotubes (DWCNTs) have been observed, leading to the formation of very long linear carbon chains. The formation and growth of long linear carbon chains is investigated by Raman spectroscopy and density-functional tight-binding-based molecular dynamics simulations. The high-resolution transmission electron microscope images of C10H 2@DWCNTs show that the encapsulated linear C10H 2 molecules have a length of 1.2-1.3 nm. After the high temperature annealing at 1073-1273 K under high vacuum conditions, new Raman bands appear at 1800-1850 cm-1, and the Raman bands arising from encapsulated pristine polyyne molecules at 2000-2200 cm-1 completely disappear. This is clear evidence that a fusion reaction occurs to form long linear carbon chains inside the 1D nanospace of DWCNTs. © 2011 American Chemical Society.