A　sensitive　electrochemical　sensor　based　on　immobilized　multiwalled　carbon　nanotubes　(MWCNTs)　and　1-butyl-3-methylimidazolium　hexafluorophosphate　(BMIM.PF6)　on　a　glassy　carbon　electrode　(GCE)　for　o-sec-butylphenol　(osBP)　was　proposed.　The　electro-oxidation　behavior　was　studied,　the　experimental　conditions　were　optimized　and　kinetic　parameters　were　calculated.　The　results　indicated　that　this　electrochemical　sensor　has　the　advantages　of　fast　electron-transfer　rate,　minimal　fouling　of　electrodes,　high　sensitivity　and　stability　for　o-sec-butylphenol.　Upon　comparison　with　a　glassy　carbon　electrode,　this　senor　would　effectively　minimize　the　over-potential　and　increase　the　electrochemical　response　to　o-sec-butylphenol.　Under　the　optimum　conditions,　the　peak　current　was　linear　to　the　osBP　concentration　range　from　1　x　10(-7)　to　2.5　x　10(-5)　M　with　the　detection　limit　of　8.65　x　10(-9)　M　(S/N　=　3).　The　proposed　method　was　applied　to　the　determination　of　spiked　water　samples　with　satisfactory　results.