We present the spectral properties of a large sample of nearly face-on low surface brightness (LSB) disk galaxies selected from the SDSS-DR4 main galaxy sample. About 12,282 LSB galaxies have been selected from the photometry database with their B-band central surface brightness μ0(B) ranging from 22 to 24.5 mag arcsec−2. About 7000 of such LSBGs have measured emission lines ([OII]3727, [OIII]5007, Hβ, Hα, [NII]6583) with the S/N ratio greater than 5σ. Their spectral diagnostic diagram of [NII]/Hα vs. [OIII]/Hβ shows that ~89% of them are star-forming galaxies, and ~11% could be classified as AGNs. The relations of μ0(B) vs. 12+log(O/H) and μ0(B) vs. stellar masses M* of these star-forming LSB galaxies show that their O/H and M* increase following the increasing μ0(B). The majority of these LSBGs are on the higher branch of metallicity.