In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 Pm were studied with the combination of differential scanning calorimetry (DSC), temperature dependent Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The main discovery is that the microencapsulated n-octadecane crystallizes into a stable triclinic phase via a mestastable rotator phase (RI), which emerges as a transient state for the bulk n-octadecane and is difficult to be detected by the commonly used characterization methods. As evident from the DSC measurement, a surface freezing monolayer, which is formed at the interface between the microcapsule inner wall and n-octadecane, induces the crossover of the R-1 from transient to metastable. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R, phase, and consequently the lower relative nucleation barrier in the confined geometry turns the transient R, phase into a metastable one.