The photocontrol of a bilayer-to-nonbilayer phase transition (the liquid-crystalline L_α phase to the inverted hexagonal H_II phase) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) by the photoisomerization of incorporated stilbene molecules was examined by utilizing differential scanning calorimetry, small-angle X-ray diffraction, ultraviolet (UV)/visible absorption, and attenuated total reflectance Fourier transform infrared spectroscopies. cis-Stilbene lowered the transition temperature, T_h, to a greater extent than did trans-stilbene, and the difference was at most ca. 10 °C. At temperatures higher than the T_h of POPE/cis-stilbene but lower than that of POPE/trans-stilbene, the photoisomerization from the trans to the cis form of the stilbene molecules by irradiation with UV light caused a L_α–H_II phase transition. The UV irradiation partially induced the H_II phase at a constant temperature because of the incomplete photoisomerization of stilbene (ca. 60%). The reduction in T_h by the incorporation of stilbenes was caused mainly by the reduction in the spontaneous radius of curvature of the lipid monolayer, R₀. The greater bulkiness of cis-stilbene as compared to the trans form resulted in a more effective reduction in R₀ and stabilization of the H_II phase.