The miscibilities of blends of homopolystyrene/styrene-butadiene/styrene (PS/SBS) and PS/SB-4A (4-arm star block copolymer) have been studied by dynamic mechanical analysis (DMA) and C-13 CPMAS NMR techniques. The results indicate that the miscibilities of these two blends depend on the molecular weight of PS (M(PS)) and are almost independent of the molecular architecture of the studied block copolymers. It is also found that there exists an optimum range of M(PS) which will give rise to well-improved mechanical properties of the blends. The reason suggested for this phenomenon is that the intermolecular entanglement interaction plays a critical role. It is also found that even when the ratio value of M(PS)/M(bPS) (molecular weight of PS block in SB block copolymers) is as high as 1.2 for PS/SB-4A and 1.5 for PS/SBS, PS is still miscible with the PS block microdomains very well. Meanwhile, when M(PS) is much higher than M(bPS), the blends of PS/SBS and PS/SB-4A are still partially miscible.