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The dependence of spectral properties of the Nd3+-doped (45- x)RO· xNa2O·2.5Al2O3·52.5P2O5 (R=Mg, Ca, Ba, 0≤x ≤31, mol%) glasses on RO and on its substitution with Na2O was investigated. Absorption, fluorescence spectra and lifetimes were measured. Both the stimulated emission cross section and the spontaneous emission probability for the 4F3/2→4I11/2 transition of Nd3+ in glass were calculated. The results indicate that the effect of RO depends on its ionic field strength and that the substitution of RO with Na2O loosens the glass structure so as to lower the degree of disorder of the phosphate chain structure. A stimulated emission cross section as large as 4.2×10-20 cm2 and half-lifetime concentration as high as 8×1020 ions/cm3 were obtained.