We systematically investigated the effect of surfactant addition on the orientational change in polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymer films as a function of film thickness and surfactant (oleic acids (OAs)) concentration using microscopic analysis, neutron reflectivity, and theoretical calculations. The orientation of PS-b-PMMA films containing OAs as surfactants was found to be determined by the competitions among the surfactant-assisted neutral field and preferential fields at both top surface and bottom interface. In the case of a BCP film containing a small amount of OAs, the integer film thickness of domain spacing L(0) (long spacing of lamellae-forming BCP) prefers to form the perpendicular orientation of microdomains while the parallel orientation is observed when the film thickness is close to the half-integers-of L(0):We also found that the periodic orientation of block domains gradually dies out to the perpendicular orientation as the film thickness as well as the surfactant concentration is increased. Neutron reflectivity analysis of deuterated polystyrene-block poly(methyl methacrylate) (dPS-b-PMMA) block copolymer thin films containing OA surfactants was also performed to gain insights into the orientation of BCP films across film thickness due to the addition of OAs. With the free energy calculations on BCPs mixed With OA surfactants at different film thicknesses, the periodic orientation of block domains is qualitatively confirmed.