A spontaneous vortex structure in magnetic domains (vortex domains) has been visualized by the decoration technique near the ferromagnetic transition temperature ( T C = 18 K) in a ferromagnetic superconductor EuFe 2 (As 0.79 P 0.21 ) 2 single crystal with the superconducting transition temperature ( T SC = 22 K). Meissner domains previously observed by magnetic force microscopy have also been observed in a narrow temperature range (Δ T ∼ 1 K) near T C . A domain structure with the alternating direction of the magnetic flux has been observed upon cooling below T C . Experimental results are in reasonable agreement with a theoretical estimate of the periods of domain structures and the temperature interval below T C in which they are observed.