The structures and electric characteristics of alkanethiol and alkanedithiol self-assembled monoloyers : (SAMs) were studied using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). We show molecularly resolved STM images of alkanethiol SAMs. The current voltage characteristics (I-V curves) of alkanethiol SAMs were also measured to investigate the electrical properties. The obtained asymmetric I-V curves were discussed in connection with the structures of alkanethiol SAMs as the electric dipole sheet. We also studied the structures of alkanedithiol SAMs with STM and found a new structure consisting of stripes. We suggest that the molecules are lying with their axes parallel to the surface. The obtained symmetric I-V curves were well explained in terms of the structures of alkanedithiol SAMs. Moreover, the STM image of the mixture SAMs of alkanethiol and alkanedithiol molecules showing the phase separated structure was obtained.