The crystalline/crystalline poly(L-lactic acid)/poly(1,4-butylene adipate) (PLLA/PBA) blend system exhibits upper critical solution temperature (UCST) behavior below the melting temperature of PLLA. When the PLLA/PBA (50/50) blend is cooled to crystallize below UCST, phase-separated domains appear prior to crystallization of PLLA, resulting in PLLA spherulites being overlapped with the phase-separated domains. However, phase-separated domains only appear in blends when they are crystallized after being subjected to maximum melting temperature (T-max) for a short melting time (Delta t(max) < 3 min); oppositely, the phase domains in blends become invisible when held for longer times (>5min). When PLLA/PBA (50/50) blend is melted for short Delta t(max), the chain entanglement density is less; thus, the PBA chains are easy to be mutually expelled from the PLLA chains during crystallization of PLLA, resulting in PLLA-rich and PBA-rich separate domains. Apparently, phase separation in the PLLA/PBA blends is initiated by the crystallization of PLLA below UCST. Fractured surfaces of bulk blend samples show crystals of a polygonal shape, which is due to impingement between neighboring spherulites growing in 3D interior. For short Delta t(max), the polygonal crystal is porous with two different sizes. Small and large pore sizes correspond to the PLLA-rich and PBA-rich domains, respectively. The formation of porous-structure lamellae in the PBA-rich domains is due to combined factors: (a) preformed PLLA spherulites acting as a growth template of PBA, leading to preferential oriention of PBA along PLLA lamellae, and (2) densification of PBA upon crystallization at ambient temperature. Detailed analyses were focused on the lamellar assembly leading to various pore sizes in the phase domains and dependence of the assembly patterns and pore sizes on the parameters governing the phase separation.