The CaS:Sm3+ nanocrystallites are synthesized by the modified sol-gel technique. The grain size of the nanocrystallites is approximately 60 nm, which is determined by transmission electron microscopy, x-ray diffraction and UV-vis absorption spectrum. Compared to the CaS:Sm3+ macrocrystallites, the absorption and emission of the nanocrystallites have a blueshift in their spectra, and the emission spectrum of the host CaS appears in the region of 400–500 nm. It was also found that the efficiencies of both photoluminescence and upconversion in the nanocrystallites are higher than those in the macrocrystallites.