The novel marine degradable poly (4-hydroxybutyrate) (P4HB) synthesized by chemical technique could obviously improve the brittleness of polylactide (PLLA) via physical blending. However, the miscibility, crystallization and morphology in PLLA/P4HB blends are still unexplored. In this article, the PLLA/P4HB blends with different weight ratios were prepared by the solution casting method, followed by the melting crystallization treatment. The PLLA and P4HB were demonstrated to be miscible near the melting temperature of PLLA as revealed by the Flory-Huggins interaction parameter calculating by melting point depression method. Anyway, PLLA and P4HB are supposed to be immiscible at room temperature according to the interfacial tension, fourier transform infrared spectroscopy (FTIR) analysis and scanning electron microscopy (SEM) observation. Both of PLLA and P4HB could form the highly crystalline crystals as casting from solution. The existence of P4HB could significantly induce the cold crystallization of PLLA on the second heating curves characterized by differential scanning calorimetry (DSC), showing of a sharp reduction in peak temperature of about 40 ^oC lower than that of pristine PLLA. The polarized optical microscopy (POM) and atomic force microscopy (AFM) observation revealed that the P4HB phase could penetrate into the region between fibrous crystals of PLLA spherulites during the isothermal crystallization at 130 ^oC, and the P4HB fragmented crystals will form after cooling to room temperature, leading to the formation of vertical phase separation in PLLA/P4HB blends associating with the surface tension data. The findings will provide theoretical guidance to understand the miscibility in PLLA based fully biodegradable blends, modulating the morphology and properties of the blends.