Photoluminescence (PL) intensity was measured from Sice quantum wells, which were grown al different molecular beam epitaxy (MBE) temperatures (300 similar to 800 degrees C) followed by subsequent annealing (900 degrees C). Results indicated that luminescence intensity was very low at a specific MBE temperature range (400 similar to 650 degrees C), in which clustering of Ge atoms is dominant in crystal growth. Theoretical calculation shows that Ge substitution for Si causes a-like symmetry in the conduction band top, but pairing of Ge atoms changes the local symmetry of Si-Ge bonds which destroys the s-like symmetry. This result well explains the fact that intense PL is obtained only in the growth condition where Ge atoms are dispersed in Si. Additional PL experiments under [110] uniaxial stress indicated that Pr, intensity was very sensitive to the applied stress field. This confirms that local symmetry of Si-Ge bonds plays an important role in the light emissivity from SiGe quantum wells.