Ultrathin films of isotactic poly(1-butene) (iPBu) with highly oriented form I fibril crystals have been prepared by a melt-draw technique. The melt recrystallization of these films after vacuum evaporation of a thin carbon layer on their surfaces has been conducted and highly oriented films with form II chain fold lamellar structure have been obtained. Therefore, the spontaneous II-I phase transition of thus prepared iPBu films has been studied and compared with their non-oriented counterparts either with or without carbon-coating. The results show that surface carbon decoration slows down the II-I phase transition of the non-oriented films with respect to the non-carbon-coated ones. In strong contrast, the II-I phase transition of oriented films with carbon-coating has been tremendously accelerated. This is attributed to the fixing effect of coated carbon layer on the extended iPBu molecular chains initially packed in the crystalline unit cell of the fibrils. These extended molecular chains pass through multiple crystalline lamellae and consequently generate external stresses between the lamellae, which have been confirmed to promote the II-I phase conversion. Moreover, the recrystallization temperature shows evident influence on the II-I transformation. The higher the recrystallization temperature, the faster the II-I transition, which has been associated to the increased external stresses originating from tightly entangled loops caused by thickening of the lamellae at higher crystallization temperature.