The energy-releasing process and the conformational dynamics of water-soluble stilbene dendrimers upon photoexcitation are studied by the time-resolved transient grating method. While the energy relaxation and the conformational change are completed within 30 ns for the first generation (W-G1), slower dynamics are observed for the second (W-G2) and the third (W-G3) generations. The enthalpy change by the photoisomerization increases and the dynamics of the conformational change becomes slower with an increase in the generation. The diffusion coefficients (D) of the trans- and cis-forms of W-G2 and W-G3 are almost the same, whereas a slight difference is observed for W-G1. These characteristic features are consistently explained in terms of the electric charges on the dendron surface.