Tin dioxide (SnO2) box beams, or tubes with square or rectangular cross-sections, are synthesized on quartz substrates using a combustion chemical vapor deposition (CVD) method in an open atmosphere at 850degreesC to 1150degreesC. The cross-sectional width of the as-synthesized SnO2 tubules is tunable from 50 nm to sub-micrometer depending on synthesis temperature. Each tubule is found to be a single crystal of rutile structure with four {110} peripheral surfaces, and <001> growth direction. Although several growth patterns are observed for different samples, the basic growth mechanism is believed to be a self-catalyzed, direct vapor-solid (VS) process, where most new material is incorporated into the bottom parts of the existing SnO2 tubules through surface diffusion. The tubes are readily aligned in the direction perpendicular to the substrate surface to form tube arrays. These well-aligned SnO2 tubule arrays with tunable tube size could be the building blocks or templates for fabrication of functional nanodevices, especially those relevant to energy storage and conversion such as nanobatteries, nanofuel cells, and nanosensors. A gas sensor based on a single SnO2 nanotubes demonstrated extremely high sensitivity to ethanol vapor.