Post emergence herbicidal activities(pI_(50)) of X-substituted phenylvinylsulfone derivatives(S) in-vivo against rice(Oryza sativa L), Barnyard grass(Echinochloa crus-galli) and Pickerelweed(Monochoria vaginalis Presl) were measured by the pot test under paddy conditions. The (S) showed herbicidal symptom rapidly with lower activity(average pI_(50)= 2.0) as proherbicide, which was excellent tolerance to rice. The structure activity relationships(SAR) were analyzed using such a physicochemical parameters as hydrophobic(π) and molecular orbital(MO) quantity by the multiple regression technique, and discussed with quantum pharmacology. The herbicidal activities were related to the hydrophobic(π) effect of X-substituent and orbital(HOMO & LUMO) energy. In case of Pickerelweed, the effect was rationalized by parabolic function of π constant, where the optimal value of π was 1.10. An increase in hydrophobicity and negative orbital energy by the electron attracting X-substituent may contribute to the herbicidal activity. Based on results proposed from SAR analysis, the mode of herbicidal action could be assumed.