We found that Ca additive effectively suppresses the photo-darkening (PD) effect in Yb-doped silica fiber even at 6.0 wt% of high Yb2O3 concentration. Ca ion works as a stabilizer to maintain the Yb3+ valence state. We hypothesize that this phenomenon may reflect the local charge balance effect in silica glass. This Yb-Ca co-doped silica fiber can be useful for high-power fiber lasers.