Based on Group Contribution Theory, a fragment constant model to estimate mixture toxicity is proposed in this paper. The toxicity (EC50M) of 58 mixtures is determined using Photobacterium phosphoreum. Analysis of these EC50M and the mole fraction of the individual chemical fragments (Br, Cl, NO2, OH, NH2) helps work out the fragment toxicity contribution (Ti) to EC50M. Thus, a linear regression equation is established between the toxicity contribution Ti and the fragment constants of Hansch fi, and this equation is so significant that it helps provide an approach for calculating EC50M.