Er/Yb co-doped soda-silicate glasses with 0.5 at.% Er and various concentrations Yb were fabricated. The absorption and the photoluminescence (PL) spectra were measured and analyzed. The PL intensity of the Er/Yb co-doped soda-silicate glasses is enhanced 4 times approximately, and the optimum Yb concentration for the PL intensity at 1536 nm is 3.0 at.%. The PL spectrum becomes broader with the in crease of Yb concentration, up to a FWHM of 80 nm at 6.0 at.% Yb. The relation b etween absorption and PL spectra, together with the mechanism of PL broadening, has been addressed.