The effects of the stereospecificity of a polymer chain and of the interaction in polypropylene (PP) solutions on the relation between liquid–liquid phase separation and crystallization were investigated by using an isotactic PP (i-PP) and a syndiotactic PP (s-PP) of high stereoregularity and of similar molar mass. Dialkyl phthalate was used as a solvent. A series of dialkyl phthalates with a different number of carbon atoms in the alkyl chain was employed to control the interaction between polymer and solvent. Phase transition temperatures were measured by optical microscopy with a hot stage. Liquid–liquid phase separation temperature (TL–L) in the system of i-PP and dihexyl phthalate was located below its melting temperature (Tm). However, TL–L for the s-PP system in the same solvent was elevated much above its Tm due to a decreased Tm and increased TL–L. The reduced solubility of s-PP is primarily attributed to enhanced hydrophobicity arising from alternate positioning of the methyl groups along the polymer chain. As the length of the alkyl chain in the phthalate increases, TL–L decreases significantly and Tm decreases slightly, resulting in the value of TL–L shifting below that of Tm for the solution of s-PP and dinonyl phthalate.