A wavelength converting includes a diffused-reflecting layer, a substrate, a photoluminescence layer, and a binder. The diffused-reflecting layer has a first surface and a second surface facing away from each other. The substrate is over the first surface of the diffused-reflecting layer. The photoluminescence layer is over the second surface of the diffused-reflecting layer. The binder is mixed at least in the photoluminescence layer or at least in the diffused-reflecting layer, the binder includes a structural unit represented by formula (1), and a characteristic absorption band in a Fourier-Transform Infrared (FTIR) Spectrum of silicon-oxygen-silicon bonds (Si—O—Si bonds) in the binder is from 900 cm−1 to 1250 cm−1, in which R represents an aromatic group.