Photoluminescence of CaS: Dy and Ca1-xLaxS: Dy prepared in Na2CO3-S and K2CO3-S flux was investigated. The excitation spectrum of CaS: Dy consists of two dominant bands at 260 and 280-300 nm. Excitation band at 260 nm is assigned to the band gap energy of CaS, and the 280-300 nm band to the excitation from the acceptor level originated from VCa(2+). By analyzing the influence of x in the spectra of Ca1-xLaxS: Dy, the peaks from f-f transition of Dy and bands from the host matrix could be resolved and assigned. Emission spectrum of CaS: Dy prepared in Na2CO3-S flux has sharp peaks from f-f transition, in addition to two broad bands at 525 and 580 nm. The broad band spectrum is associated with defects and vacancies of host material, that is, the peak at 525 nm is from the defect pairs of MCa(2+)3+-VCa(2+) and VS(2)-VCa(2+) and the peak at 580 nm is from the pairs of MCa(2+)3+-NaCa(2+)+ and VS(2-)-NaCa(2+)+. The sharp emission peaks at 486, 573, and 665 nm could be assigned to the transition of 4F9/2→6H15/2,4F9/2→6H13/2, and 4F9/2→6H11/2 of Dy3+, respectively. However, emission spectrum of CaS: Dy prepared in K2CO3-S flux, shows only the f-f transition of Dy3+. The dependence of the emission intensity ratio, Iyellow/Iblue, of CaS: Dy on Dy3+ concentration and flux was explained.