The modification of the Stranski-Krastanov growth mode produced by the strain-field from buried islands has been non-destructively investigated by photoluminescence (PL) spectroscopy in lattice-mismatched Ge/Si system. The samples consist of double Ge layers with each coverage of 4.5 monolayers separated by a Si spacer layer with various widths. At an intermediate Si spacer thickness of 100-200 Angstrom, the modification of the growth mode of the second Ge layer was clearly evidenced in the PL, spectral feature which was characterized by the superposition of two different pi, spectra. In addition, a drastic increase of the density of Ge islands in the second layer was observed by atomic force microscopy. These facts show that the strain-field from the buried islands is very important to control the growth mode of lattice-mismatched semiconductor heterostructures.