We investigated theoretically the effects of molecular rotation on the STM tunneling conductance for three types of σ-bonded molecules, i.e., (a) bicyclo[2,2,2]octylmethylthiolate (BCO), (b) cyclohexylmethylthiolate (cHex), and (c) 1-pentanethiolate (C5), which are anchored to the Au(111) surface by Au-S bond. In the case of BCO and cHex, the conductance does not change when the molecules transform from the most stable to metastable configurations. In contrast, in the case of C5, the conductance is found to decrease markedly, which means that transformation from the all- trans to gauche conformation can work as a switch between ON and OFF states. This finding would give a possible mechanism of the conductance switching recently observed in the STM images for the C5 molecules inserted into the BCO SAM matrix. [DOI: 10.1380/ejssnt.2004.186]