We performed quadrupole mass spectrometry (QMS) of Si clusters that thermally desorbed from various surfaces in ultrahigh vacuum. To investigate the effect of substrate on cluster formation, different kinds of substrates were prepared : Si(100) wafers, Si(111) wafers, and Si-deposited Ta polycrystalline sheets. When Si wafers were heated at 900-1300 degrees C, QMS spectra showed that clusters up to Si-6 sublimed from the surfaces. Both of (100) and (111) had the same activation energy for desorption of the clusters. On the contrary, QMS spectra from Si-deposited Ta at 1500 degrees C showed monomeric Si only. These results agree with the thermodynamic consideration that the desorption rate of each cluster is determined by its formation energy. The relation of cluster formation with the surface structure is also discussed.