Mixed-phase areas are produced in highly elongated BiFeO3 (BFO) thin films as a consequence of strain relaxation. A (001) neodymium aluminate (NdAlO3; NAO) substrate (a∼3.747 Å) prominently suppresses the strain relaxation effect and prevents the formation of mixed-phase regions. This creates a pathway to the thick, quasipure, highly elongated phases required for magnetoelectric applications. We characterize the crystal structure, the interface between film and substrate, the surface morphology, and the ferroelectric domain structure of BFO films on NAO substrates and compare them with those of films on typical lanthanum aluminate substrates. The underlying mechanisms are discussed based on the intriguing nature of phase competition in bismuth ferrite phases using first principles density functional calculations for the misfit strain-dependent total energy.