In this paper, the confined crystallization and phase transition behaviors of n-octadecane in microcapsules with a diameter of about 3 microm 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 (R I), 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 I from transient to metastable. We argue that the existence of the surface freezing monolayer decreases the nucleating potential barrier of the R I phase, and consequently the lower relative nucleation barrier in the confined geometry turns the transient R I phase into a metastable one.