The effect of substitution of Sr2+ by Mg2+ on the luminescence properties of (Sr0.9 − xMgx)Si2O2N2:0.1Eu2+ were investigated as a function of the concentration of Mg2+(x). The peak position of the yellowish green emission from (Sr0.9 − xMgx)Si2O2N2:0.1Eu2+ was invariant with x. However, the luminescence intensity was significantly affected by x. With increasing Mg2+ concentration, the intensity increased to a maximum at x = 0.1. Partial substitution of Sr2+ by Mg2+ enhanced the intensity 1.8-fold. The Eu2+ concentration affected the peak positions of both luminescence and excitation. With increasing Eu2+ concentration, the red-shifting and enhancement of the luminescence were associated with the nephelauxetic effect. Concentration quenching of the luminescence occurred above y = 0.07 in the (Sr0.8 − yMg0.1)Si2O2N2:yEu2+. Above the critical concentration of Eu2+, non-radiative transitions between adjacent Eu2+ ions were more effective than the nephelauxetic effect. Optimized (Sr0.83Mg0.1)Si2O2N2:0.07Eu2+ phosphors were fabricated with blue GaN LED and the chromaticity index of the phosphor-formulated GaN LED was investigated as a function of the wt% of the optimized phosphor.