An agent for improving lawn grass quality, which is excellent in promoting dwarfing of lawn grass, making the stem sturdy, increasing the leaf count, and improving greenness, is provided.An agent for improving lawn grass quality containing, as active ingredients, 5-aminolevulinic acid represented by formula (1): R2R1NCH2COCH2CH2COR3  (1) wherein R1 and R2 each independently represent a hydrogen atom, an alkyl group, an acyl group, an alkoxycarbonyl group, an aryl group or an aralkyl group; and R3 represents a hydroxyl group, an alkoxy group, an acyloxy group, an alkoxycarbonyloxy group, an aryloxy group, an aralkyloxy group or an amino group, or a derivative thereof or a salt of the acid or the derivative, and a gibberellin biosynthesis inhibitor.