Calcium silicate cosubstituted with Sr2+ and Eu2+ was studied in order to examine the relationship between emission characteristics and Eu2+ occupancy at two types of Ca sites (Ca(1n) and Ca(2n)). The results clearly indicate that red emission of the Ca2SiO4:Eu2+ phosphor is caused by Eu2+ substitution at the Ca(2n) site. In addition, red shift in emission of Eu(2n) occurs when large amounts of Eu were substituted.