The diameter of individual single-walled carbon nanotubes (SWNTs) was successfully modulated along their axes by instant temperature control in a laser-assisted chemical vapor deposition (LCVD) process. SWNTs were grown using different temperature profiles to investigate the effects of temperature variation on their growth. Due to the inverse relationship between SWNT diameter and growth temperature, SWNTs with ascending diameters were obtained by reducing the LCVD temperature from high to low. The diameter-modulated SWNTs were grown across a pair of Mo electrodes to form field-effect transistors (FETs) for investigation of their electronic transport properties. Fabricated devices demonstrated properties similar to Schottky diodes, implying different bandgap structures at the ends of the SWNTs. Raman spectroscopy, transmission electron microscopy, and electronic transport characteristics were studied to investigate the influence of temperature variation on the structural and electronic characteristics of SWNTs.