The energy-releasing process and the conformational dynamics of stilbene dendrimers (tetramethoxystilbene (generation (G) 0, G1, G2, and G4) upon photoexcitation are studied by the time-resolved transient grating method. For all dendrimers, the excited triplet (T-1) state formation was observed. From the time-resolved studies on energy and the molecular volumes changes, it is found that the conformational change completes with the decay of the T-1 state for G0-G2. However, the dynamics is slightly slower for G4, which is attributed to the conformational change of the dendron part. The molecular volume change due to the conformational rearrangement of the dendron upon the trans to cis isomerization is slightly positive (molecular expansion). The expansion is explained in terms of the creation of the void volume surrounded by the dendron side chains. Although the diffusion coefficient (D) of the trans and cis forms of GO is almost the same, the difference becomes larger with increasing generation. This is clear evidence for the large conformational change upon isomerization. The larger D of the trans form compared with that of the cis form is explained in terms of the surface roughness of the dendrimers.