N-Phenyl oxadiazolidinedione derivatives II were synthesized and their herbicidal activities were measured against grass weeds. A parabolic relationship between molar refractivity (MR) of meta substituents of dione Ⅱ and their herbicidal activities was observed. With the substituents having MR value=${sim}15$, the higher activities were obtained. Especially, the highest herbicidal activity (97% inhibition of weeds at 0.25 kg/ha) was observed by propyne IIr containing propargyloxy group as meta substituent.