We demonstrate a distinct confinement induced cooperative lattice orientation change in ultrathin P(VDF-TrFE) films by using various characterization techniques, including X-ray diffraction (XRD), two-dimensional X-ray diffraction (2D-XRD), grazing incidence X-ray diffraction (GIXD), and infrared reflection absorption spectra (IRRAS). Both the polymer chains and the molecular dipoles that are perpendicular to the polymer backbone showed a lying-down orientation change with decreasing thickness. The dipole orientation change showed a good correlation with the thickness dependence of remnant polarization, which were measured from highly reproducible ferroelectric loops in a wide range of thickness (30-250 nm) on inert electrode with suppressed "dead layer" effect. A simple microscopic molecular dipole interaction model revealed that the free energy of different orientation states was related to the thickness and lateral dimension of the polymer crystallites. The findings reveal a unique molecular orientation driven size effect in ferroelectric polymer films, providing new insights into the nature of ferroelectricity and orientation mechanisms in polymers relevant to the design of emerging flexible electronic devices.