The temperature dependence of C-H stretching modes of alkyl monolayer formed on Si(111) surface was investigated by an attenuated total reflection Fourier transform infrared spectroscopy from room temperature up to 540 K. Continuous disordering of the monolayer was indicated from the gradual peak shifts toward higher frequency in C-H stretch modes upon heating. The irreversible conformational disorder was introduced in the monolayer above 440 K.