In one-dimensional (1D) systems, the definition of three common states of matter (solid, liquid, and gas) becomes obscure because it has been theoretically predicted that a 1D system has no phase transition. Due to technical difficulty in tracking 1D thermal evolution, hardly any experimental evidence has demonstrated whether there exist these three states. Here we report Raman experimental observation that ID iodine molecular chains formed inside the nanosized channel undergo continuous transformation from chain structure to single molecules with increasing temperature, without having a sudden change as commonly observed in phase transition. At low temperatures, short-range order exists and manifests itself as long chains in structure, which gradually break into shorter chains with increasing temperature. The ID system progressively gets more and more disordered, which is in agreement with the theoretical derivations. Our work may benefit the emerging molecular scale electronics.