The thermal expansion of La0.5Sr0.5Co0.25Fe0.75O3 (LSCF55) is investigated both by first principles phonon calculations combined with the quasi-harmonic approximation (QHA) and by experimental approaches. Within the framework of the QHA, the volumetric thermal expansion coefficient of rhombohedral LSCF55 is calculated as (V,GGA) = 50.34 x 10(-6) K-1. For comparison, the lattice expansion and the volume expansion of LSCF55 grain are measured by in situ high-temperature X-ray diffractometer (HT-XRD). An anisotropic thermal expansion of rhombohedral LSCF55 with (a,hex) = 10.89 x 10(-6) K-1 and (c,hex) = 21.18 x 10(-6) K-1 is obtained. The volumetric thermal expansion coefficient is measured as (V,HT-XRD) = 43.17 x 10(-6) K-1. In addition, the effectively isotropic expansion coefficients of a polycrystalline LSCF55 bar specimen are measured using a vertical high-performance thermo-mechanical analyzer and yield (l,bar specimen) = 17.37 x 10(-6) K-1 and (V,bar specimen) = 52.11 x 10(-6) K-1.