The crystallization behaviors and microstructures of poly(L-2-hydroxyl-3-methylbutanoic acid)/poly(L-lactide) blends [P(L-2H3mB)/PLLA] were investigated by OM, DSC, SAXS, in situ temperature-dependent WAXD, and in situ synchrotron WAXS. The blends exhibited a homogeneous state at 250 degrees C. In the cooling process, P(L-2H3MB), with higher melting temperature, crystallized first at 166.7 degrees C and drove the formation of the phase separation and microstructure. And the amorphous P(L-2H3MB) and PLLA were excluded into the interlamellar and interfibrillar regions of the former P(L-2H3MB) crystallites. For P(L-2H3MB)/PLLA (5/5), the, amorphous P(L-2H3MB) in the interfibrillar regions continued to form crystallites sequentially at 134.6 degrees C, which clearly confirmed the fractional crystallization of P(L-2H3MB). In our knowledge, this unique fractional crystallization has not been reported yet, of the component in miscible blend, which crystallized first under little confinement. This work provided a new viewpoint and understanding of the relationship between fractional crystallization and microstructures in crystalline/crystalline blends.