We report on the fabrication and electrical characterization of aligned multiwall nanotubes (MWNTs) grown on a four-probe patterned catalyst layer. This structure has been designed to directly measure the electrical property of as-grown MWNTs. The temperature-resistance results show that the aligned MWNTs are semiconducting in directions perpendicular to the tube axis and follow the three-dimentional hopping conduction mechanism. Effects of oxygen plasma on the characteristics of the MWNTs are also investigated. Raman spectroscopy results indicate that oxygen plasma treatments can be used to reduce the carbonaceous material in the film. As the exposure time of oxygen plasma increases, the resistance of the aligned MWNTs increases mainly due to the suppression of current conduction through carbonaceous materials. These results suggest that oxygen plasma treatment is effective in improving the film quality of as-grown MWNTs. (c) 2005 American Vacuum Society.