We studied the atomic and electronic structures of a one-dimensional (1-D) junction with low temperature scanning tunneling microscopy (STM). The junction smoothly connects two parts of single wall carbon nanotubes (CNTs) introducing defect structures. The two parts of the carbon nanotube are both semiconducting with the measured band gaps of 0.92 and 0.80 eV In order to map out detailed electronic structures, we performed scanning tunneling spectroscopy (STS) measurement at the selected positions on the axis of carbon nanotubes. We observed gap states localized near the junction. (C) 2004 Elsevier B.V. All rights reserved.