Silicon quantum dots fabricated by nanosecond pulse laser in nitrogen, oxygen or air environment have enhancement in photoluminescence emission. The stimulated emission was observed at about 700 nm. It is difficult to recognize the difference between the photoluminescence peaks from samples in different environments, which is because of the same structure of the electron states in the band gap for different samples. The calculation results show that the same structure of the localized states forms in the band gap when silicon dangling bonds on surface of quantum dots are passivated by nitrogen or oxygen. It is the localized states that could catch the electrons from the conduction band to form metastable states, which is the key factor to enhance photoluminescence emission.