1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer with stilbene was investigated by using phase-contrast and fluorescence microscopy, differential scanning calorimetry, and Fourier transform infrared and UV-visible absorption spectroscopy. The results indicate that the temperature of the gel liquid-crystalline phase transition of the DPPC bilayer with cis-stilbene is much lower than that with trans isomer. Photo-irradiation to trans-stilbene in the DPPC bilayer suppresses the pretransition (L-beta' to P-beta'), while the gel-liquid-crystalline phase transition (P-beta' to L-alpha) changes little because of the poor photoisomerization rate in both gel and liquid-crystalline phases. It is shown that the photoisomerization of incorporated stilbene and the phase behavior of the lipid bilayer depend on each other.