We present a sol–gel derived coating optical fiber sensors for measurement of critical micelle concentration of surfactants. A sol–gel process for making porous silica and coat the fiber core with sensing material is described. CMC detection is based on an adsorption effect in sample solution mainly of sodium dodecylbenzenesulfonate. Three different particles size of sol–gel porous cladding were tested. Test results show that the sensitivity of the small particle size porous sol–gel cladding optical fiber is higher than that of large particle size sol–gel cladding one. It was also found that measurement of CMC point using this method is very accurate and cost effective.