Iron oxides with various compositions and polymorphs have been widely used as compounds that require reversible redox properties, such as catalysts. However, partial decomposition during phase transitions often causes irreversible degradation of the redox properties of iron oxides. Cr doping into the crystalline framework of iron oxide dendrites improves the stability of the structural transformation of iron oxides. We spatially visualized the FeO_x-dendrite phase distribution during oxidation in crystalline dendritic FeO_x and Cr-FeO_x particles by full-field nano-X-ray absorption fine structure spectroimaging. The spectroimaging visualized propagation in the phase transitions in the individual FeO_x particles and changes in the phase transition behaviors of the Cr-FeO_x particles. The statistical analysis of the spectroimaging data revealed the phase transition trends in parts of the FeO_x and Cr-FeO_x particles in three Fe density zones (particle thicknesses) and the probability densities of the phase proportions in the dendrites.