Abstract: In a previous report⁽¹⁾ the determination of partition coefficient with a pellicular chemically bonded silica gel (Permaphase ODS) was investigated. The different log P ranges are measured using different mobile phases: 60% methanol-water covers 3.37～4.45, 30% methanol-water 2.11～3.37 and 1% methanol-water 1.58～2.13. The microporous bonded octadecylsilane supports (10μm Partisil ODS and GYT-C₁₈) have good efficiency on separation. In this paper, we investigated the determination of partition coefficient using Partisil ODS and GYT-C₁₈ packing column and compared the results with Permaphase ODS packing column. The experimental data showed that in one mobile phase system(75% methanol-water)the capacity factors could be determined in the log P range of 0.90～4.45. In this case it is not right for the purpose of determination of log P. But within narrow range of log P, such as 1.39～2.69, log P correlated with log k' linearly. As compared with Permaphase ODS packing column, the deviation is large. Therefore, the determination of partition coefficient using a pellicular bonded octa-decylsilane support is more suitable.