A novel atomic force microscope (AFM) having a tip on whose surface sensor molecules were immobilized has been developed for chemical sensing. The AFM tips chemically modified with octadecyltrichlorosilane [CH3(CH)17SiCl3; OTS] or perfluorotrichlorosilane [CF3(CF2)7C2H4SiCl3; FS-17] were more sensitive than the unmodified tips in detecting adhesive force on some monolayers on the silicon substrates. Using OTS modified tip, the hydrocarbon chain length of chemically adsorbed alkyltrichlorosilanes [CH3(CH)nSiCl3, n = 1,8,13,17] on the silicon substrates was discriminated in ethanol environment. The origin of the interaction between the chemically modified tips and the sample surfaces, which depend on the monolayers and environmental liquids where the interactions were measured, has been speculated to arise from the entwining of carbon chains in monolayers on the tip and the sample.