Self-assembled alkanethiol monolayers (SAMs) were prepared on the Au(111) substr ate. The behavior of the transport properties of SAMs under external load was in vestigated by the conduction atomic force microscopy. The current of SAM was fou nd to increase with the external load, and this property can be well characteriz ed by the Hertz model. It was also found that the conductance of SAM decreases e xponentially with the chain length of the alkanethiol molecules. The correspondi ng current decay constant β is demonstrated to decrease with the external load at first, and then becomes independent of the external load. Furthermore, the cu rrent of SAM with long chain molecules increases more apparently than that with short ones. These features all originate from the tunneling effect via nearby mo lecular chains for the charges.