Effects of chemical composition and structure of unsaturated polyester (UP) resins on the miscibility of styrene (ST)/UP/low-profile additive (LPA) ternary systems containing thermoplastic polyurethane (PU), poly(vinyl acetate) (PVAc) and poly(methyl methacrylate) (PMMA) as LPAs, at room temperature and cured sample morphology were investigated. Three series of UP resins were synthesized, which included maleic anhydride (MA)-neopentyl glycol (NPG)-diethylene glycol (DEG) types with varied molar ratio of NPG and DEG; MA-propylene glycol (PG) types with and without modification by a saturated dibasic aromatic anhydride or acid, such as phthalic anhydride (PA) and isophthalic acid (IPA); and MA-PA-PG types modified by a second glycol, such as DEG, 2-methyl-1,3-propanediol (MPDiol), and NPG, to partially replace PG. Ln addition, based on the MA-PA-PG-DEG type of UP, other UPs with varied degree of unsaturation for C=C bonds and molecular weight, respectively, were synthesized. The dipole moments, mu, for UP and LPA have been calculated on the basis of group contribution methods. Finally, the miscibility data of ST/UP/LPA ternary systems and the characteristics of cured sample morphology have been explained by the calculated polarity difference per unit Volume between UP and LPA.