A simple method of calculating log P (partition coefficient in octanol/water) has been developed based on the quantitative structure-log P relationship for 1230 organic molecules having a wide variety of structures. The 1230 organic compounds investigated included general aliphatic, aromatic, and heterocyclic molecules together with various drugs and agrochemicals. The predictive structure-log P model obtained by multiple regression analysis involved only 13 parameters for hydrophobic atoms, hydrophilic atoms, their proximity effects, unsaturated bonds, amphoteric property, and several specific functionalities. A saturation effect was recognized in the parameters for hydrophobic and hydrophilic atoms, and unsaturated bonds. The structure-log P relationship was highly significant as the F-statistics=900.4. This simple method appears accurate enough for semiquantitative uses in structure-activity rating studies, especially for quantitative structure-activity relationship in toxicity.